I recently asked a similar question: Determining Water Change Schedule
Do you have a good skimmer?
Do you have vegetation, such as chaeto, and are they growing well?
Both contribute to removal of undetectable dissolved organics and other pollutants in the water.
It also depends if your system is overstocked or understocked (such thing is a rarity in this hobby), nitrAte levels in between water changes, and the amount of food you feed your fish. In a well established system that is not overstocked with good skimmer and vegetation, I'd think you can push out water changes to ~30-40% once per 3-4 weeks. More frequently if nitrAte start to rise.