It's different for every tank, for several reasons. We do water changes to remove bad stuff and add good stuff. The amount of bad stuff accumulating in the water is diff for each tank, as is the amount of good stuff being consumed. Plus diff setups vary in their tolerances for how much bad is ok, and how much good is needed.
So what you do is monitor the tank. You set a goal for bad stuff like nitrates and phosphates, and see how long it takes them to creep up over where you want them by testing the water. You watch your glass to see how long it takes to build up some algae. You watch your animals to see what they look lik when they are happy, and notice if they start to look weird (like your anemone scrunching up because the water is dirty). And you set a goal for good stuff like alk, ca, mg, and see how long it takes to start dropping too low.
Then you just change the water as often as necessary to keep everything on track. I like 10% every week until the tank is really stable (6 months-1year) then every other week. But it totally depends on your tank. Besides age, theres lots of variables like what else you are doing to remove nutrients (skimming, vacuuming sand, filters) and what you are doing to add good stuff (salt quality, dosing, kalk topoff). So the balance between adding and removing is really individual and its best to start with a normal, rule of thumb schedule and go from there.
