Ditto what SK said - it depends -ALOT - on the tank, the fish, the plants, the water parameters (especially KH and GH, in my tanks), etc.
My big guys (125g mixed south american cichlids) generally get 50% per week. Sometimes more.
Their tank has a thin sand bottom, and I've got a reef-style maxi-mod pump on there (mad mad flow) so nothing really collects on the sand - so I don't vac that tank, I pump the water out instead.
In one of my planted tanks, I let the detritus settle into a mulm layer on the bottom - the plants seem to do well that way, and it helps provide infusoria for the fish that are breeding in there (I learned this trick from a local apisto breeder). So that tank also doesn't get vac'd - but it does get about 50% changed every 1-2 weeks.
However - I work 2 jobs and have over a dozen tanks, plus help run a fish club and am webmaster for a ferret shelter - so I don't get to do as many water changes on each tank as I'd like.
So for my other tanks, its a bit more casual.
I have some killie breeding tanks that get water changes "when I remember" -- they are
extremely densely planted, and I'm not actually
trying to breed the killies, they just won't stop
Ditto for my Lake Tang. shellies - I know I get more fry when I change water more often, but its been more like every 3 weeks lately.
I've got a 20g tank that I recently found ancistris fry in - but I honestly don't remember putting two ancistris in there in the first place - so I wasn't exactly cranking out the water changes (it was a killie tank). But they still decided to surprise me with fry.
However - all of my "slightly less often" tanks have either a relatively low bioload, a high plant load, or both.
Vegatative filtration can do alot toward keeping disolved organics in check.
All that being said - I usually recommend people change some water about every 2 weeks.
Personally I prefer to change about 50% weekly. Its just not realistic right now for me.