It depends on the stocking.
Most reefers aim for a balanced ecosystem with a skimmer and some macroalgae to remove nutrients to make water changes almost optional.
Your nitrates, phosphates and other levels will determine just how often you have to do a water change. I know of some setups where there hasn't been a water change in 6 years, and plenty of people do one oevery 6-12 months.
On the other hand, keeping SW preds in a FOWLR will probably want at least 10% per week.
