If you want stable parameters, do it more frequently. If you want more efficient use of your time, water and salt, do it less frequently. So depends on what you want to compromise, it can be anywhere from a continuous water change (if you live right by the ocean with access to clean seawater) to some of us who do it every other year or longer. Another parameter is the % of water change. It really depends on the type of things you keep and how you take care of them. More feeding, you will need more frequent water change with large %, etc.. The rule of thumb is that if you are not sure, you probably not doing it often enough.