It is going to depend on your bioload, how heavily you feed and if you are skimming. If you have a generally light bioload and light feeding, you can get away with 10%-20% every 2-3 weeks. If everything is on the heavy side, then I would do 25% every 2 weeks. Try a couple of different methods and see what results you get. If you try 10%-20% every 2 weeks and the tank is getting dirty fairly quickly, then switch to 25%+ every 2 weeks. I for one used to change 25%-30% every 2 weeks in a 60g. HTH