When I think to check or when they get dirty, whichever comes first. I can usually hear the difference when they start to clog.
Usually 4-5 days, sometimes sooner.
Mine's setup so that when they are saturated the water is diverted over a spillway so whenever that happens I change them. Usually 5 to 7 days. Toss 'em in the washing machine with a splash of bleach and then let them COMPLETELY dry.
Here is a something worth looking at. I'll try to find the original article I read when I was told to remove my socks and let the skimmer/refugium do it's thing.