Customs clearance at Newark vs. Dulles

Do you happen to have Global Entry?
Are you going to book the flights together on one ticket or are you trying to book them separately? That makes a big difference--if booked together, the airline is going to give you a "legal" layover time, estimated to be enough for the transit. I would not want to venture a guess on an appropriate amount of time for a separately booked itinerary--it would be several hours in case of delays in addition to the retrieve-baggage-and-recheck rigmarole.

I used to live in San Francisco and fly United to Europe (usually to Paris) -- sometimes nonstop and sometimes with connections in Newark, Dulles or Chicago. The worst for me was Newark by a mile in terms of wait time and not very nice workers and IMO a lousy AirTrain system to get between terminals so in your situation, I would go for Dulles. Here is a report from The Points Guy that goes over the wait times at the airports and information on how long the waits average depending on the date or travel and the time.

If you don’t have any expensive purchases to declare you just exit the area. Passport Control/Immigration will take longer. If you obtain Global Entry it will not much time at all.

What Global Entry gives you is high confidence that you won't wait in a line to
get through immigration (not customs). I cannot speak for IAD or EWR, but at
ORD I never have to wait for a Global Entry kiosk, and I am out into baggage
claim in about 5 minutes. Otherwise, it depends on the lines, which depends
on how many inbound international flights are coming in around the same time.
As already noted, Customs is a non-factor unless you declare items and have
to pay duty, or are caught smuggling something illegal.

Given the airports you've noted, I assume you're on United, and if all on one ticket,
you'll be taken care of in the event of delays. You have to claim your bags at
the intl baggage claim area if you checked any, and that can take time. If you
just have carry-on, then the main risk is whether your inbound flight gets delayed.

If by chance your flights are separately ticketed, then the game is quite different.
While the procedure is the same, if you are delayed and miss the second flight,
you're at the mercy of the airline.

It’s so hard to predict. We returned through Newark in July and there was no line at immigration. I had MPC ready to go but it was unnecessary because there was no line for anyone.

If you don’t have Global Entry, I do recommend getting the MPC app. It has saved us time. And it’s free.

My wife is mobility challenged and travels riding a folding portable scooter. She goes to the front of every line, and I follow towing a pair of carry on bags. We seem to most often avoid the long ques.

We just returned via IAD a few days ago. Global Entry was fast, the non-GE lines were long. Just pay attention to get on the right shuttle for connecting flights, not final destination.

The benefit of flying United and connecting through IAD to RIC, is doing immigration and customs in the mid-field arrivals hall at the C/D Terminal. With Mobile Passport Control I've gone through the process in a matter of minutes and then pop back out in the C/D Tterminal for the next United flight.

If you have Global Entry, you can download the Global Entry app and skip the kiosk. It’s available at Dulles, I’m not sure about other airports. It functions like the kiosk. You input where you are landing and other information and take a selfie. Hopefully you get the magic green check and go directly to the Global Entry immigration officer. I check in while I’m waiting on the people mover to go to the main terminal.

Thank you, everyone! Unfortunately , I didn't know to book both legs of my trip on one ticket so I'm not protected that way. I will apply for Global Entry and hope it comes through before I fly in April. Does anyone have a guess as to how long it takes to get to the United connecting gates at Dulles once you clear immigration and customs? We come from Paris on United.

Dulles intl arrival can be slow going if multiple flights land at once. That's why I got Global Entry for only one flight per year--it's well worth it! The people movers are maddening.
I will venture a wild guess and see what others say: If I was doing this, I'd want at least three hours, for leeway for a delayed flight, lines, and waiting for baggage (if applicable).
When you buy the second flight, spring for something that is changeable in case you are late and miss it.

Longtime Continental/United customer. Transited Newark many times, and Dulles is now my primary international airport.
You need to understand the Global Entry application process. It is a great thing to have if you travel (or plan to travel) internationally often. However, it is not a short simple process. I just want to make sure you understand, and realize you might NOT be able to get it before your trip.

1. apply online
2. receive "conditional approval"- I received mine in days, my husband's took 2 months. They say it can be longer.
3. Schedule an in person interview at an airport (without having to be flying anywhere). You can do this at many airports around the country but the interviews can be very hard to get. We tried for 3 months with no luck. We were finally able to do it as an "interview on arrival" coming back in to Newark. Unfortunately if your connection is tight you probably won't have time to do it at Dulles.
4. Are you flying out through Dulles? They have a pilot program there for "interview on departure" in terminal B. If you have time there on your way out of the country that is probably your best bet. As far as I know Dulles is still the only airport for this.

Once you are approved in interview you are good to go for 5 years, and it includes precheck status.
Make sure to check what documents you need for the interview. They are particular about documents to prove your address.

If you don't get Global Entry? All is not lost! As said above, use MPC- Mobile Passport Control. Download the app. Set up your profile (picture of passport, etc). On arrival back on US soil, go back to the app. It will ask for a selfie, a few questions about your travel and then give you a QR code to take to the immigration officer with your passport. There should be a separate line in the immigration hall for MPC but you may have to look for it or ask a helper. I've come back into Dulles twice this year and while the regular immigration lines were long- MPC only had a couple of people in line! Prior to getting Global entry I used MPC and it saved well over an hour and a half at Toronto airport. There are signs everywhere in the immigration hall about MPC but seemingly most people don't pay attention and don't realize how much time they could save. Honestly, until more people start to use it, it can be as fast as Global Entry.

If you have global entry or use MPC, waiting for you luggage will be the slowest part at Dulles. If it is separate tickets what airline to DEN? If it is United the connecting immigration is at terminal C/D so not a long walk. If another airline it might be a different terminal. (or in my case I sometimes take a small regional United plane which is out of terminal A). Anything other than terminal C/D will take more time. If your flight is on time and you use MPC or GE and depart from terminals C/D you may be fine with 1.5hr connection. I generally book a minimum 2.5hr connection and have plenty of time to spare, but I'm risk averse.

Still better than Newark :)

Thank you for that very detailed and helpful answer!
I just booked my return through Dulles.

Customs is a walk-through unless you have purchases to declare. So no worries regarding customs whichever airport you choose.