Personally, I think it will differ from tank to tank how often a change is needed all depending on the fish, water quality, tank size, substrate, feeding schedule and filtration (biological, mechanical, etc).
I do a 30% change via gravel siphon in my 20 gallon unplanted every 6 weeks or so (a little more frequently in winter due to increased evaporation, maybe a little less frequently in the summer months). And I do top off with fresh water as needed in between changes.
Now I know some people are gasping out there, but it's not as terrible as it sounds. I test the water every week - no ammonia, no nitrites, and nitrates are under 5 ppm range. My pH is stable at 7.6. GH/KH is in the 125-150 ppm range (acceptable for the fish I keep). My fish are happy and healthy and living normal domestic fish lives. I've never had any problems with ich or other diseases since I established the tank 4 years ago - and some of the danios and platy I used to cycle the tank originally are still around.
Is this schedule appropriate for all tanks? No. I think careful weekly testing of all water parameters is the best way to figure out your tanks water change schedule.