Looking on line I find water changes of 20-30 % weekly seems to be the average from sites like Aquatics and Practical fishkeeping for a well stocked tank. Most sites state fewer fish means less water needs to be changed and not as often. Also the size of the tank makes a difference. Currently I do a 15-20 % weekly water change with 21 small fish. Several sites stated that every two weeks is fine with a planted tank depending on the number of fish and size of tank. Others talk about small daily changes and one talked about 90% weekly change. Some sites caution that changing too much water can cause shock or injury to fish. I had looked at maybe 20 sites or more before sitting up my tank. Don't get me wrong I appreciate your input, it just that there is a lot of info on-line saying different things.
Yes, this is indeed a good discussion to have, and I certainly have no problems discussing water changes. The long-held and often-repeated "myths" metalhead refers to are confusing especially to the less experienced; in my more than 30 years in the hobby I have accepted or refudiate them one by one.
The more water you change the healthier all fish will be. Fish in nature are in "new" water with every respiration they make, and we cannot even come close to this in an aquarium unless we have flow-through water.
It is true that more fish means more frequent and/or larger volume water changes are mandatory just to (hopefully) keep them healthier, but the aim should always be healthier fish, not finding the minimum and ending with that. There is a lot of "minimum" this or that in the hobby, but being satisfied with just the minimum is really not a good approach to keeping any animal.
The volume is especially significant; changing 70% of the tank once a week removes far more pollution that would changing 10% every day, even though the weekly volume is exactly the same. Most of us do not want to be doing daily water changes, so once a week is minimum (any less frequent than this will be detrimental to the fish). Beyond that, trying to relate volume to the number of fish is rather pointless because one is really only trying to get out of doing it.
Jack Wattley used to write in his monthly TFH column about water changes. He noted that many discus breeders will change 90-95% of the water in fry tanks not just once but two and some three times each day. The result was faster growth, and healthier fish. That in itself demonstrates the immense benefit of water changes.
Fish are producing pheromones and allomones regularly, and these can only be removed with water changes. There are other chemical substances as well, along of course with organics. Relying on water tests for high nitrates before changing water is not going to benefit the fish, quite the opposite.