up to you. The more frequent, the better I say. Weekly is tops. Others will tell you differently. Even give you math equations to prove their point (Hawk...:bowdown:). In the end, it is up to you and what keeps your tank healthy. The more you stock your tank, the faster it gets messy and the more often you need to do water changes and the bigger the water changes. A good rule of thumb is 10% of your tank when you do a change. You can probably get away with every two weeks if your tank is lightly stocked, but do weekly when your tank is fully stocked.
-Dr Marco :sfish: