I do about a 50% change twice a month, so every 2 weeks(this includes gravel vacing, srubbing decorations and scrubbing side of tank, and changing filter material enless it doesn't need to be changed). This has worked great for me and the fish, because I need to change atleast 50% of the water to get a good bit of the gunk out while gravel vacing, and the fish are all healthy, water params are great.
Edit: Oops.. ) I see this is for BETTA tanks, not all my other tanks. In that case, I change them about every week in a half, but my divided tank (5 capartments, 18 gallons) is almost ready, will have fish in there tommorow, so I'll be doing daily water changes (mostly to get poop and uneaten food out) and a 100% water change whenever needed (not too sure yet how often I'll be doing this).