If you don't have a large amount of life in your tank (in relation to tank volume), you won't have a lot of detritus. If you don't have a lot of microalgae or bacteria, your socks will also stay relatively clean.
With a heavy bioload, 200 micron felt filter socks get full to overflowing for me in 5 days (with no sand stirring); most recently I have been changing them out every 3-4 days. They are deep brown throughout. I get tons of stuff out by handwashing, and then I follow with a hot machine wash. No bleach. Air dry. Even without bleach, clean ones typically cause my skimmer to foam a lot.
Just last week, I decided to switch to 200 micron mesh filter socks. Although nominally the same pore size as the felt socks, in reality there is no comparison. The mesh socks let far far far more material through. I expected this and in fact hoped for it; I would like more microscopic flora and fauna to circulate, and see what it does for my livestock, if anything. And I can do without the skimmer foaming which accompanies the felt socks. Also, I'm really sick of cleaning those felt socks. Cleaning the mesh socks is easy, just takes a few minutes to clear the crud that collects at the bottom of each. :)