Because of the underachieving skimmer/filtering/return pump that came with my 29 gallon biocube, I have to do a 10% water change at least once a week. I just look at the water and it looks like it's just begging for a change.
So I would say the frequency depends on how well your filtration system and circulation is working. But i'm a newb to this. Just my 2 cents.