Newbie Algy Question

[pendergast]

Sorry another question i will probley have millions as i go on. I got told to change out my filter spunges every 3rd to 4th time i clean my filter, Im worried about how often i should clean out my filter as i dont want to do it to much or to little. It is a fluval 303 canister 3 stage filter, spunge, bio, then carb. My guess was every 3 weeks never thought to ask anyone about it.