how many hours in lake plitvice?

I can't tell you what is the best hike; I can't even tell you which route I took through the park on my most recent visit nine years ago. You can see a lot in 5 hours. I'd guess you'll have at least an hour, maybe closer to two hours, before the hordes descend. Get into the park as early as you can so you maximize your time there when it is uncrowded.

I'd really, really recommend that you get to the park by mid-afternoon, though, so you can see some of the park that day as most of the other visitors are departing. If you do that, you won't need as much time the next morning. It won't necessarily be easy to predict when you need to head back to your hotel to retrieve your luggage and then to the bus station if you're potentially fighting your way along packed walkways. The walkways are narrow, and they are two-way, so getting past others can be nearly impossible. In most cases there aren't any railings, so trying to move three abreast could easily dump a bunch of people into the water.

im there early may before the season starts so the bus schedules are more limited. What I also could do is go straight from Split to Zadar and then the next day to a day trip to lake plitvice. It's an organized one and we're scheduled to leave at 8 AM, arrive at 10 AM, do hike C which is supposed to be one of the longer ones that includes the upper and lower lakes, and then we leave at 4 PM and arrive back at zadar at 6. So it's 6 hours at the park instead of 5 but the problem is that it's when all the crowds are (10 AM to 4 PM instead of lower crowed (8 AM to 1 PM)

Five hours is probably, ultimately, the amount of time I spent at the park though it was the typical late afternoon/morning visit. I’d suggest visiting the lower lakes at entrance 1 (farthest from the hotels) first. The first section of this hike starts high above the valley and takes you to some of the most dramatic falls. There’s a point where you get past the falls and it’s more lakes, or you can hike back up to entrance 1. Hike back up. If you do this keeping an eye on time it should be about two hours. Take the tram back to the hotels and the upper lakes. You’ll take a quick 2 minute boat ride across for this section. This trail is a little longer perhaps, but you can stay on pace. It’s mostly flat. You’ll have to really watch your time, but you’ll at least be by the bus stop and not reliant on tram service at a critical time.

awrzesinski, my lodging is at a guesthouse that is something like a 10-15 minute walk from entrance 1

Generally there are 2 interesting areas, the upper lakes and the lower lakes, which are connected by bus and boat (or on foot, but if you're short on time I wouldn't recommend this).
Entrance 1 is at the lower lakes, entrance 2 is at the upper lakes.

You may check out the following website to prepare yourself and find out what is most important to you.
https://www.earthtrekkers.com/how-to-visit-plitvice-lakes-travel-guide/

If you decide for either the upper or lower lakes 5 hours are plenty. If you would like to see both areas it might not be enough time. The problem is that you can't go at your normal pace on a crowded narrow footbridge with lots of people.

In this case I would try to be at entrance 2 at 8am (maybe there is a bus or cab). Then you can start right away with the upper lakes and then take the shuttle bus to the lower lakes.
Here you now have the option of being flexible. If you have little time and it is very crowded, stay on the upper path towards entrance 1 and enjoy the view from above. If time permits, you can walk down by the lake towards the large waterfall.

I would avoid the organized tour visit, and arrive on your own as soon as the park opens. Once those tour buses arrive, the crowds become insane. Our visit was mid afternoon and the walkways were packed with people and dogs. It was a wonder that no one ended up being pushed in to the lake. As mentioned previously, walkways are narrow and attempting to pass slow people can be frustrating. If I remember correctly, our tickets had a one hour arrival time window. We purchased them online in advance so that we did not have to wait in line at the park.

In that case OP, I’d flip it and see the upper lakes first. At the lower lakes, the most dramatic part is the kind of the first half of the hike. I feel like if time really got short you could experience this section a little faster. The upper lake trail is more of a closed loop. My biggest concern is getting you close to the bus stop when you’re done, you don’t want to miss that!

If you could hitch a ride from a friendly traveler or if folks at the guest house would drive you to the upper lakes entrance before it opens that would be ideal. I was the first person in there, and ended up being the only person on the whole trail. The early morning sun hitting the falls up there was just incredible.

thanks so much acraven, awrzesinksi, paul and travelbug.

i spoke with my lodging and they said "In the morning, you can take your luggage with you because there is a luggage office at the entrance to the park. The entrance to the park is at the bus stop" so that should save me some time. I just pre-buy my ticket, show up at 8 to store my luggage (i assume what they said is accurate, if not, that screws everything up), and soon thereafter start. I will end up having something like 5 hours 30 minutes for the park with a 15 minute grace period to make sure i have time to make the bus and everything. The organized tour is 6 hours but it will be a lot more full at 3 PM than 8 AM.

I'm also there May 1 or around there so crowds shouldn't be close to peak yet.

As far as route, I think I'll try for route 'C' which starts and ends at entrance 1 and the internet says is 4-6 hours which presumably includes additional time due to crowds.

Sounds like a plan, though I’d still be a little nervous not being in walking distance of the bus stop at the end.

Yes, I'd allow more time than that to get to the bus. I had a bit of a problem locating the bus stop, and when I got there, the posted departure times for the buses didn't match what I expected; Balkan bus schedules are a bit uncertain and often difficult to pin down. I was heading back to Zagreb, which I think has more frequent buses than Split, so it wasn't a problem for me, but it could have been. If Plitvice is the last stop on that bus route (I don't know that it is), I wouldn't expect the bus to leave early, but you never know. I once barely caught a Spanish bus because I just happened to be at the pick-up point 8 minutes early. Getting stuck at Plitvice when you have a hotel room in Split would be very awkward.

acraven,
what bus did you take?
I'm buying a ticket in advance on flixbus. I assumed that means the bus time will be accurate but maybe not?

I traveled from Zagreb and back on some random Croatian bus line--or quite possibly two different random Croatian bus lines. Flixbus is a major player internationally, but it doesn't seem to have a sterling reputation for being on time. Just in general, it has seemed to me that buses are less reliable than trains; they can be affected by traffic in a way that trains are not--which is not to say that trains don't have issues sometimes as well. It's best to go into this sort of excursion with the recognition that it may not go completely according to plan. Most likely it will be fine. At least it's not going to cost you a lot of money.