Maximum early check in to Int’l flight

Assume you mean checking luggage and not just a boarding pass. We routinely check-in at the Denver airport as early as four to five hours without any problem. When do you want to check-in?

Maximum of 4 hours is common. Here is what SeatGaru says:
https://www.seatguru.com/airlines/Delta_Airlines/checkin.php

Yes, you may check-in for your Delta Air Lines (DL) flight at the
airport ticket counter from 30 minutes to 4 hours before the scheduled
departure time and receive a printed boarding pass.

I do mean checking in with luggage, not just getting the boarding pass. Nowadays I usually get and print the boarding passes on line 24 hours in advance. We are considering checking in the luggage 4 hours (if that is the maximum time) in advance and returning to my home 20 minutes away for relaxing dinner and bathroom before returning closer to flight time with our carry-ons. Would prefer to then try to sleep through the 7 hour 45 minute flight, arriving early morning.

You need to provide more specific info (like what time is your flight and why do you want to do this?).

Assuming you are talking about checking in at the airport (as opposed to online), it depends. There needs to be staff present (and willing) at the check-in counter. At a major airport (JFK) with a major airline (Delta), the counter might be staffed 24/7, but it depends on their schedule. Do they have flights going out all night long? If not, you may need to wait. If there's nobody at the Delta counter until 7 am, there's no point going there at 3 am expecting to check in.

That seems like a lot of work to save a couple of hours. The reason we check in very early in Denver is because we have engaged in some time shifting and are on a time pattern for the evening meal around 3,4pm. We go to the airport early enough to have a leisurely dinner with wine at one of the better airport restaurants. With a 7.30, 8 pm departure we are ready for sleep within an hour of wheels up. Helps a lot with jet lag are the other end.

Most airlines will not accept checked luggage more than 4 hours before scheduled departure from what I have experienced. I suggest you ask them what they allow. They have limited space in the luggage processing area and if everyone dropped their luggage very early they would simply run out of storage space. Also, luggage laying around many hours before flight departure could easily end up being left or being put on the wrong plane.

Since you are also requested to be at the airport 3 hours before international departures, your plan would not allow for much of a dinner at home if you can only drop your baggage 4 hours before departure.

"You need to provide more specific info (like what time is your flight and why do you want to do this?)."
Flight is 7:30 pm, and we want to go home to relax and eat dinner before, and then return and sleep through the 7 hour 45 minute flight if possible.

"Since you are also requested to be at the airport 3 hours before international departures, your plan would not allow for much of a dinner at home if you can only drop your baggage 4 hours before departure."

But once we check in with the luggage 4 hours before, how do they know if we then leave the airport and return in time for boarding? There is no second check-in although we do have to go through security with our carry-ons.

I do mean checking in with luggage, not just getting the boarding
pass. Nowadays I usually get and print the boarding passes on line 24
hours in advance. We are considering checking in the luggage 4 hours
(if that is the maximum time) in advance and returning to my home 20
minutes away for relaxing dinner and bathroom before returning closer
to flight time with our carry-ons. Would prefer to then try to sleep
through the 7 hour 45 minute flight, arriving early morning.

JFK is my home airport (as is LGA).

Ok, for a 7:30pm flight, you are considering checking in at 3:30pm, go home (20 minutes? You must NOT need the Belt Parkway!), relax at home and then return to JFK. What time did you think you would be returning?

Unless you have TSA Precheck, you should be at Terminal 4 security line no later than 5pm. Your int'l flight could close 30 minutes prior to boarding time which for you is around 7pm.

So given the parameters, I don't see how your plan could work. What I would recommend is that you either print or put your boarding pass on your mobile phone (download the Delta app if you haven't already) up to 24 hours in advance, arrive at T4 at 4:30, drop your checked luggage with a Delta agent at a Delta kiosk, and then go to security and wait on line.

But once we check in with the luggage 4 hours before, how do they know if we then leave the airport and return in time for boarding? There is no second check-in although we do have to go through security with our carry-ons.

Going through security IS the second check in. The TSA scanner for your boarding pass is tied into the airline's systems -- you get scanned at the security checkpoint, the airline knows you are there. Of course this doesn't apply to connections where you don't go through TSA check, but that is not the case here. Just reiterating what every airline requests for arrival time. You are of course free to ignore that and arrive when you want. I don't fly through JFK so I don't know what the security lines are like or how long it takes to get to a gate after security.

I live about the same drive time from my hometown airport as well. Before TSA I would have done something like this. I did it a couple times in Las Vegas, years ago. But now I would rather only make a single drive there for any trip I am taking and simply relax in an airport lounge. The hassle with parking, unexpected traffic delays, who knows how long it will take to check luggage -- simpler just to go once and be done for me at least.

"Unless you have TSA Precheck, you should be at Terminal 4 security line no later than 5pm. Your int'l flight could close 30 minutes prior to boarding time which for you is around 7pm."

My understanding is that once we checked in with our luggage, the flight cannot close on us so long as we appear at the gate for boarding time at 7 pm. We have TSA Precheck, but are you saying that is may take up to 2 hours, from 5 to 7 pm, to get through Terminal 4 security without the TSA Precheck?

If you are just trying to ditch your luggage, relax, eat some dinner and use a (semi) private bathroom before you board your flight so you can just go straight to sleep once on your plane, why not just buy a single-use pass to the Delta Sky Club? Or (if it applies to you), I think American Express has a Centurion Lounge in NY (might be LGA - you'll have to look it up). Even if you don't want to eat the food Delta Sky Club puts out in their lounges, you can always just get something you really like from a local restaurant before you head to the airport and just eat it in the lounge. That way, you aren't having to make 2 trips to the airport.

Thanks all, and especially Ciedleh who gave me the best idea!

We have TSA Precheck, but are you saying that is may take up to 2
hours, from 5 to 7 pm, to get through Terminal 4 security without the
TSA Precheck?

Yes. I know that line well unfortunately....that is until I got Precheck. You have it which is very helpful. I would then arrive by 5:30; if one of the xrays is down, even the Precheck line could take time.

FYI, if you have an AMEX platinum card, the entrance fee is waived for you to enter and enjoy the Delta Sky Club located on Concourse B by Gate 31. . There is no AMEX Centurion Lounge at T4.

get something you really like from a local restaurant before you head to the airport and just eat it in the lounge

Actually, no you can't according to their rules. They will throw you out if you try and they notice. I have seen it when a family brought chicken nuggets for their 2 yr old to eat during a long delay and they were forcibly removed (the cops were brought in when the family refused to leave, there may be more to that situation than I saw).

https://www.delta.com/content/www/en_US/traveling-with-us/airports-and-aircraft/delta-sky-club/house-rules.html

Whoah! I certainly wouldn't want to have Delta call the McNugget Police on me. My friend has Sky Club membership and I've been ticketed on same flights as her and she's been able to get me in for free. We've sat and eaten things we brought in from elsewhere (she's a pretty strict vegan - so finds it hard to eat something decent she doesn't either make herself or get at a place that specializes in vegan/organic). No staff members from Delta approached us any of the times we did that, but maybe we just are really lucky - or maybe they just ignored it because my friend logs a ridiculous number of miles with them and perhaps that info pops up on their screens when she checks into their lounge.

Still, if OP is perhaps willing to eat the Sky Club food, it could still be a viable option for them. And some of the airport restaurants are not that bad now. I also wanted to skip eating the airplane dinner on a flight from Boston to Amsterdam last Fall. I was able to have a sit down dinner at at Legal Seafoods right in the terminal and then when I boarded the plane I just put my headphones on and tuned out everything around me until they turned the lights down on the plane.

The rest of the story is that we only eat Kosher food. Hence the dilemma. It is served on board but apparently not in advance even in the Sky Club Lounge.

I suppose we can bring our own, eat in the general food court area, and then go into the lounge.

I remember once reading about kosher food at JFK. The article is two years old so you may want to check to see if it's still available.

Kosher Food at JFK

For potential future reference, how will you keep Kosher in Madrid and Spain in general?

I suppose we can bring our own, eat in the general food court area, and then go into the lounge.

You can easily bring your own food, but just remember that you'll have to pass through TSA before you reach the other side of the airport. They will look at liquids.

I don't know if you require a specific hechsher, but I did pull up this which appears fairly recent

https://www.star-k.org/articles/articles/travel/3557/star-k-kosher-certified-food-options-take-off-at-jfk-laguardia-and-newark-airports/

Thanks to all for your responses and helpful information. In response to Lo's inquiry, there are kosher restaurants, and supermarkets with kosher food items, in Madrid, Malaga and Barcelona, as well as Gibraltar which we are also visiting. Otherwise, we will take-out certain items with us from those places; and many basic items such as fruits and vegetables, are kosher everywhere. In addition, we will be served a kosher meal on the AVE train Preferente from Madrid to Seville.