What are your readings? If you plan on it being a reef tank you want to make sure your nitrates are low, so base you water change on that. Once you have everything under control you can then get an idea of how much and how often you need to do water changes.
what test kit do you use ? i do weekly 5g water change's on my 24g ap with sump and dsb is about 30g total, also when i had my 62g i only did weekly water change of 5g. alot applies to how heavy the tank is stocked, type of corals and how good the system takes care of its self to a degree