I change about 20% 2 times a week I usually clean the sponges on my filters at the same time in the dirty water. You'd be surprised at how dirty they get!!
When I was first begining and having a lot of problems I was only cleaning my filters about once a month. Once I started cleaning them more often I started having less and less problems. The dirtiest sponge is in the 26 gallon which houses my bristlenose pleco. Man when I squeeze out the sponge the water goes BROWN!! It's gross
15% twice a week. A 5 gallon (19l) plastic carboy is easy to manage and in my 125 litre tank that works out to 15%. I'm considering doing larger changes though, 30% twice a week.