Good question! I'm wondering that, too. I've read 2-3 months or as often as every month, but I use top soil and peat; thick layer, in 4'x4' tables. And it seems I have successfully, tho inadvertently, litter trained my guys. They only eliminate in water; their drinking water. (Little buggers! <SIGH>)
Btw, I was warned never to use sand, as it's known to cause impaction. I'm not sure if this applies to all species--I have Hermann's torts--so wait and see what the experts recommend.
Most keepers only spot clean, and only replace if there's trouble afoot (unwanted bugs, etc). Some keepers have great results in throwing in some pill bugs (rolly pollys, sow bugs) and worms to breakdown wastes, eat unwanted bug larva, and keep the soil "fresh" -- I doubt those keeps ever replace substrate fully.
You'll get different advice on sand, with a strong preference towards "not at all" and "only a small amount of sand in the mix".
Yea it's def a small amount. I got a mulch type ordeal from a pet store i use too an make it thicker. I've noticed sand was bad but also read Alittle is ok