I don't have a specific schedule, its minimum once a month of 50% change on my lightly stocked 20G but sometimes more often depending on detritus buildup on the bottom of the tank. Never 100% change as some people do but sometimes upto 75% change.
The fish are happy and healthy and have been with me for 3 years now, so I must be doing something right!
the 90G F.W tank is 50% biweekly and sometimes 2 50% changes in a row, so closer to 75% changes once a month.