My rule is a weekly 10% water change. For my current system, however, it's more than necessary, so sometimes I'll slack off and go a month without doing a water change. I think your post expresses a very important point. Our systems can be so different as far as livestock requirements and how much we feed, that having a definite rule is almost impossible. While it can be prudent to follow a rule-of-thumb, I've found that many times it's necessary to find out what the requirements of our systems are and base the water changes on that. A 50 gallon tank keeping only Xenia will be able to go much longer w/o a water change than a 100 gallon tank filled with SPS and tangs. Unless that 100 gallon tank is running a denitrator, phosban, and a calcium reactor. There are so many variables...