How often should i change the substraight?

Dunno, I used my other side as a viv so I have isopods and red wigglers in the substrate so I don't plan on it until I add the drainage layer in a few months. If you get a decent micro-ecosystem going on you should be safe as is just doing spot checks.

When it starts to smell, dirt gets too wet (soaked), or as a safe route - change every 1-2 months.

I second the when it smells. Once it looses its coco-fiber smell and gets a bit sour its time to change it. You have a large enough tank to build a viv and let it become self-sustaining

I would change it once a month and spot clean otherwise.

Ditto what the others said....also, I have that exact same shelf unit in my room, lol.

I change the substrate locally when for example the frog stayed at the same place during some days or made poops at this place.
Otherwise, I change it every month or 2 months.

The greater the enclosure size, the longer you can wait for changes.

I've changed my substrate twice in 2 years. never had a problem. My frogs only poop in there water. that gets changed daily