If all your water parameters are going well you should still do a 25% water each week.
Keep in mind the heavy or bigger your fish stock the faster these water levels change. Keeping alot of fish meants alot more cleaning.
It also depends on the size of your tank, bigger tanks need less maintainence which means less water changed, whereas small tanks need alot more attention.
Regular water changes promotes breeding also.
In my 40g breeders i don't hesitate changing 30-50% of the water weekly.
Start with 25% until you are confident in what you are doing and in the end you will no exactly what you are doing and what needs doing.
There is also a good buffer recipe in the library on this site that you should have a look at, it looks complicated but it's actually really simple and helps keep the water just were the fish need it
DJRansome gives good advice. Watch your nitrates and go by that (try to keep them below 20 ppm) but do at least 25% change weekly reguardless because they like fresh water.
My 55g. tank is heavily stocked and my nitrates get up to 40ppm so I do a 50% change weekly which brings them down to 20ppm then I do another 25-50% change midweek. I have to admit there has been times that I've missed doing the midweek & weekly change and the nitrates have got over 40ppm without any noticable harm but it's certainly advisable to try to keep them under 20% as much as possible.