I do approx 40-50% once a week unless more is needed (son dumps the whole can of fish food in to feed his fishies..or I didn't take the fruits or veggies out soon enough or sick fish or something like that).
I do more for my betta tanks because they are small and I feed twice daily. They get every other to every 3 day 100% water changes, except the ones with eco substrate, they get about 90% water change at the same intervals as above.