I try for weekly changes, but i also know my community tanks at least can go 3-4 weeks if needed, though i rather they didnt. Betta tanks dont have a choice. Always once or twice a week on those. They seem to open up for finrot after that, at least in a 5 gal tank. Maybe it is because i am a firm believer that anything less than half the tank is almost a waste of time and effort, so 50-60% is a minimum/maximum.