The idea of the water changes is to keep the substrate clean & keep the NitrAte level down. Ideally NitrAte should be at zero, however in reality this is nigh on impossible, so most people try to keep it below 40ppm.
So your water changes should be aimed at keeping below this figure. Use your test kit to start with to establish how much water you need to change to keep this figure down. Then after a while you will know what % of water keeps you nitrate below that........