Well, that would vary upon the system. In a FOWLR
system you would only be aiming on keeping nitrate/phosphates low and it is possible to do fewer water changes if the levels stay low. In a reef system, you are also replenishing other things such as calcium and magnesium that your coral use to grow. It is more imparative that you do them weekly in this type of system. But either way, water changes are the most effective way to remove nitrates.