best bet get yourself a good test kit. Learn to use it properly. I use nitrate levels to help establish my water changing schedule. Nitrates under 40 ppm is what seems alot of people go with. I have different levels I keep in different tanks though; My 85g with discus I keep below 10 ppm this is two 30 gallon changes weekly. My 75 gallon with convicts gets 15-20 gallons changed weekly but the nitrates are in the 20-30ppm range.
if you are changing roughly 10 gallons every 4 day you are probably ok, but to be sure you must test. As your fish get larger though the water change volume will need to increase.