10% weekly would be an absolute minimum in an understocked, heavily planted tank.
If you are running an average 10 - 50 gallon tank with an average fishload (too many fish, but everyone does it), plan on a 25 - 30% weekly water change.
It is not uncommon for people to do 40 - 50% per week with amazing results - better growth, more activity, better colors. larger spawns, etc... A water change dilutes the fish waste and replaces electrolytes lost to biological processes. Just like a river is always flowing, the fresher the water, the healthier the fish.
Just be certain your tapwater is suitable for fishkeeping - I.E. remove chlorine/chloramine with a good treatment, try to match the temperature, or go a few degrees cooler to simulate a heavy rainfall, and if possible let it sit for a while to let the dissolved gases out so they don't bloat your fish (most people miss this step)!
More is better, with very few exceptions. Make it a regular routine, and after a few weeks it'll hardly seem like work anymore.
Good luck and enjoy!
/Kris
