It's recommended to do a gravel vac & 25% water change weekly. There is a good stickied thread at the top of this forum about how to perform weekly maint.
I do two water changes weekly. Wednesday is a 20% water change with light gravel vac. Saturday I do a 30-40% water change with deep gravel vac, trimming plants, removing hard water stains, checking the filter for debris, etc.