The answer to this will vary as it depends on how often you go to the loo, and how many people are using it. The easiest way to tell is with an RV tank sensor, installing one of these will easily alert you when your tanks are full and need emptying. Otherwise you will have to intimately know your tanks and get a sense (or smell) for when they need emptying. It’s always recommended to empty your tanks after every camping trip, so the wastewater isn’t festering over long periods.