I have to admit that I'm pretty slack... I typically do one a month, unless testing shows levels too high in nitrates and phosphates (meaning detectable on my non lab grade Elos/Salifert/Seachem tests). Obviously undetectable doesn't mean zero, but I don't fear high numbers.
I should really do better.
I usually change between 15-20%.
Now that the poll is up, I voted. Sorry I typed a whole explanation.
I'm do 30g's bi-weekly on my 120g. This is my 5th tank and just invested in a RODI less then 2mths ago...:doh:...
IMO,The most effective filtering method... Period!!!
I do about 15% every week. When My 60 cube was up. I'd do 10 gallons per week. Now with the 93, I play on doing 15 gallons per week. I seem to get good results with this routine.