A lot depends upon tank size and bioload. In general, smaller tanks and tanks with higher bioloads (fish, etc. per gallon) tend to depend more heavily upon water changes for maintaining water quality. So a 25% change, weekly (or bi-weekly for a lighter loaded tank) would not be unreasonable for tanks in the 10-20 gal range. Larger tanks (hundreds of gallons) may be more like 10% monthly (especially if not heavily loaded).
Also, watch your tank inhabitants. If things seem to noticibly "perk up" after a water change, perhaps you should shorten your period between changes.