Both of my tanks get 40-50% water changes at least once a week. More often if I get the chance. Every month or two, I do two consecutive 50% water changes on both of my tanks as well.
i try and get two 25% changes in a week, if i can't i do a larger one around 40%. then every other month i do two 50% after each other. it works for me and i've had no problems.
I have gotten into a routine of doing 25-30% changes every Saturday, plus an additional change every other Sunday. Works well for me, and seems to keep my fish happy.