Greetings and welcome aboard.
As mentioned it all depends. For me it's a combination of what corals and fish are being kept . My large tank 285g total I maintain with water changes only. Corals are softies with 4 or 5 LPS. I have a quite a few large fish that are fed heavily. I don't think I could keep this tank running without changes due to the waste from the fish. My acro dominant tank is 65g total and I dose AFR. I do minimal water changes to vacuum the detritus that settles in the back corners. I change 2-4g every 7-10 days depending on how much detritus I have to remove. I only have 4 fish but two are tangs that I feed well so they add a lot of waste. If I didn't have the tangs I could probably run this tank on AFR only since it runs pretty much as a ULNS naturally.
