It depends on what's in your water. On average, I'd replace carbon every 2 weeks when used "actively" (water forced through it like a canister filter).
Also depends on type of carbon you are using. Some cheaper brands will clog with organics in a couple of weeks, then start releasing nutrients like phosphates/nitrates. I use Kent, which guarantees that it won't release nutrients even if it's clogged to capacity organically. You might guess I'm not religious about carbon changing so I like the Kent stuff personally. The rule of thumb seems to be 2 weeks for a change or only using when water quality dictates necessity.
