I've left .308 rounds in for two days by accident before (went on trip forgot to turn off tumbler). no problems. same with pistol bullets.
I read an article in the last couple of years about this and the writer said he tested all sorts of ammo, factory and reloads, all different powders etc etc. and had no problems, and no increase in pressures or velocities. I know that the last time I mentioned this someone posted a quote from some other, older test that did show an increase when stuff was left in for a long time. One of the caveats that the newer article mentioned was that he was using new, modern powder which is much more durable. It was also mentioned that he thought, but couldn't prove, that stick powders might eventually break down and cause some problems. but again he couldn't get it to happen.
however I usually only leave my rifle stuff in the final tumble for maybe 15-20 minutes. Pistol usually 1/2 an hour or so, though an hour isn't uncommon.
if you think about the amount of vibration that happens with finished rounds, before they are ever fired you'll see that leaving them in a tumbler isn't a big deal. everything from bouncing around in a car trunk to the vibrations from being in a magazine during firing, all of these things don't affect the rounds, and neither will time in a tumbler to clean off the last bit of goop from loading.
although I guess it's slightly possible that someone could use a powder the guy I read didn't use and they could have a problem. if you don't feel safe doing it, don't. I do so I do.