How long is it safe to keep a cat in a carrier?

We are driving cross country for a move. We've been stopping after ten hours of driving, though, because I wasn't sure how long it was safe for them to stay in the carrier without food/water/bathroom. However, this is taking a very long time! Since there are two drivers we could go 15-20hrs a day with no problem. How long is safe to keep them in the carriers before we stop for the night? They are all 6 years old and in good health. The careers do have beds in them.|||When I moved from Texas, I had a an extra large carrier that had blankets and a small litter box in it and a food dish and water dish. That was a 18 hr drive with three cats in there. They did fine.|||it is hard to tell, each cat is different, for instance i have two cats, one will ride with you where ever you go and will wait to till you stop to go, the other will wait maybe an hour and then freak out, none are the same|||well i brought my kitten from the west of ireland across the country, across the sea on a ferry to england and then a 3 hour drive home with little worries, i didnt use a cat box but a large cardboard box with blankets, water etc|||do NOT let your cats out at a rest stop to stretch they will run away / since there are 2 people in the car put a litter box in the floor and water bowl and ever so often let them out (one at a time ) to see if they need a drink or to poop,(don't let the cat get on the driver)and keep the windows up/ and always make sure before you open your car door that your kittys are in their carriers.