You should still change the towels every day. They shed their skin often (they eat it so you won't see much just laying around) and pee a lot. You wouldn't be able to tell just by looking though. But both of these things can lead to bacteria growing, which doesn't take long at all. Also, fungi can take awhile to grow into a structure you can see, so it's probably there even if you don't see it.

An pac can stay for a good while in a 5 gallon without getting stressed, but a 10 gallon would be better for an adult.