Your water change schedule will be determined by your bioload. A lightly stocked tank might be ok to do a small water change every month, a very heavily stocked tank might need a big water change more than once a week.
Typically you do water changes to keep nitrate below 40ppm. Be careful doing a big water change though if you havent been keeping up with them. If water quality has deteriorated due to lack of maintenance and you try to rectify with one big water change then the sudden change of water parameters can stress the fish and cause issues.
Do you know your water parameters?
Useful flowchart from aquarium co op for determining your water change schedule.