I have found on my cross country trips that there are plenty of small towns scatters across the US to make finding a hotel within an hour easy (though I'm sure you can find exceptions). I have also found a lot of interesting "small town" parks and culture because I forced myself to stop and find something (anything!) to do after 11 hours on the road.
Nevada, Wyoming, Montana, possibly the Dakotas, are principle exceptions in the lower 48, though this means an upper bound on this interval, at least along principle rotes. An example is the 75 miles along I-94 between Buffalo and Cheyenne, Wyoming, with no highway exits or onramps at all. There are, however, services, at either end.
Alaska is a domain unto itself.
On the train, you will often have to pull over to let freight trains pass you.
The biggest constraint is "civilized" sleep, which is best had in a real bed at a real hotel, whose locations are constrained by settlements and whose typical availability hours begin at 15:00 local time and end at 11:00 local time. When you're driving west and passing through timezones, you can gain an extra hour that you can use for rest, while if you're driving east, you're racing doubly against the clock, as you're losing an extra hour you could be sleeping in a hotel at every timezone.
My longest "map-time" drive sprint (not including stops) between sleeps is 19 hours, which was filled with delirium towards the end, and I would never recommend. I've done a 16-hour sprint to similar results, and have done multiple ~12-hour sprints with no issues whatsoever. In my experience, 12 hours is around the upper limit of how much "map-time" distance one can safely drive between sleeps, especially if one is to perform a comparable drive for three consecutive days.