May be a stupid question...

[brock lesser]

Personally for all the reptiles and amphibians I've worked with (in rescue's at work) and at home (BP). I was always pretty rigorous about spot cleaning, and did a full clean (replace substrate) every 30 days / once a month.

Gives you a good chance to check up on everything, and rework anything that needs to be done at the same time.

Totally agree with this, also depends on your substrate it may need 2X a month, or more.
What are you using?