How long can a cat "hold it?"
12 noon to 5 a.m. the next day is my answer.
We left our old home in CA at noon, en route to the east coast, got stuck in road construction hell, stopped for take-out dinner, detoured 58 miles through the Sierras in the dark on logging roads because of more d@mn road contruction and finally hit Winnemucca at 5 a.m. Twice we made "box stops" where we set up the litter box in the trailer we were pulling, put the cats in it. No way, no how, you should have seen the look we got from one of them in between him turning to glare at the 18 wheelers zooming past.
However, once we hit the hotel, litter box set-up was job # one, and both cats (only had 2 then) used it multiple times in the first 20 minutes.
I had talked to our vet in advance about the issue, and she said to offer the cats the box, but not to be surprised if they didn't use it on the road.
There's a super jumbo sized Ziplock that holds a regular plastic litter box for our road trips, but doubt that it would work for air travel.