If you do a drainage layer and introduce springtails or isopods into your viv, you can go a year or more without changing the substrate. I've had my viv built and planted for 3 years and have only changed the substrate twice. Once because a frog died and I wanted to make sure everything was clean, and another time because all my plants died and I was worried something was off. Springtails or isopods will clean up any frog poop or dead bugs that end up in your viv floor and it all works together to make fertilizer for your live plants and a self sustaining vivarium. I only clean frog poop off of leaves, branches or glass. The rest gets left. I do change the water every day using distilled dechlorinated water.

I'm pretty sure there is instructions somewhere on this forum on making a drainage layer. Personally, I do a layer of hydroballs, a layer of fiberglass screening, and then a layer of substrate, coco fiber or plantation soil. I have a pump in the layer of hydroballs to pump out any water that gets in there, but you could just put in a tube to siphon the excess water out. I know some people drill a hole into the tank if it's nontempered glass and drain that way.