Micron rating will determine how often you need to change them out as well as how long they are (change due to overflowing not because of things breaking down in them). I currently have 100 micron on mine and have to change it every three days but it is currently very low in terms of stocking. I would keep that in mind when deciding which you go with. The other thing is there is a difference between mesh and felt. People say the mesh can go a little longer before overflowing but I found it to be the opposite for some reason.
It is one of the things in the system I look at when I switch it out and think I cannot imagine leaving the system without it (much like the skimmer cup) because of the crap in it.
A lot of people use the bleach, as mentioned above, but I can't get myself to use it. I know there has been no issue but I would be the unlucky one so I let the sock sit out for 24 hours, soak it in peroxide for about 20 minutes, throw it in the wash hot cycle, then let it air dry. It gets them looking nice and clean without any worry of chemicals being trapped.