honestly i don't change it as often as i should. Every 1 - 2 months.
But I recommend making sure there are at least a few different islands for the frogs. They don't really seem to enjoy being in the water, they almost always choose to be on land. I did have one however that would actually swim down to the bottom of tank and sit there for a little while at a time.
But that reason might be why the fish and crabs have survived for so long is because the frogs don't typically spend a ton of time in the water.