Depends on the maturity and performance of your biological filtration (live rock, sand, bio-balls, etc.) combined with your bio-load and how heavily you feed your tank. Best bet is to change it every 2-3 days as a starting point, and then dial in the frequency for your particular tank.
Watch the floss chamber to ensure the water level doesn't 'back up' at all from floss blockage, and measure your NO3 and PO4 regularly (at least once if not twice a week) to see how low and stable they are over a period of several weeks.
If they stay low and stable, try bumping your floss replacement out to once very 4-5 days, then monitor floss chamber and nutrients again for a few weeks, and so on until you find YOUR tank's nutrient capability level.
For example, my 24g only needs the floss changed out once a week. In a pinch - e.g. if I'm traveling - it can even go for two weeks. But if I use a 2-week schedule all time, I start to notice slightly quicker film algae growth on the glass and slight rise in nitrates.
So, because of its particular setup, biofilter, and bioload, 1 week is the 'sweet' spot for that particular tank. If I change it more frequently, I'd be wasting $ on floss unnecessarily. Less frequently and I'd start to see nutrient levels rise.
In a nutshell, start with a 2-3 change out schedule, then gradually extended it to find the 'sweet spot' for your particular set up.