I think you will find very different information from almost every person on this forum; along with many great links to many great aquarists who also have different views.
The reality is this: In the wild, fish continually experience new, fresh water. The more you change it, within reason, the more close to "natural" as your fish will experience.
I change 25-30% weekly - that includes a gravel vacuum to help make sure that there is not excess waste in the gravel causing nitrates etc. My fish are very healthy, very active and my water levels are very good (0 Ammonia, 0 Nitrite and <10ppm Nitrates).
There is such a thing as too little water changes, but rarely too much. In my personal perspective (having 2 kids, third on the way, working full time and spouse working full time), the best water change schedule is one that provides a stable, healthy environment for your fish and that is consistently doable for you.