I agree with Yvonne , I have some substrate that has been in the habitat for a couple of years. That's why we use orchid bark or cypress mulch they work so much better then the other stuff, they hold moisture and don't break down so they create humidity. I put hay on top of the soil in 3 corners of the enclosure...Good luck and stay on the forum where you can get good advice...and we can follow the progress of your little fellow...
I would need to top up the hay or replace it little at a time becuase sometimes the poop is soft and messes up the hay. I will do a full swap once every 2-3 weeks to play safe.
If you spot clean (pick up all pieces of the hay that have any messes), you may find you can go a couple months. Any way, it should be obvious to you when it looks or smells icky. This is purely dependent on your substrate, tort, size of enclosure vs. size of tort, and your cleaning habits.