I change mine about every 3-5 days. I'll turn it inside out, rinse off any coarse particulates in the sink, and just toss them in the laundry & wash them with whites. Since mine have the plastic rings, I'll hang them to dry instead of putting them in the dryer. The dryer probably wouldn't hurt them, but..... never know what kind of oils/chemicals might be released by heating the plastic, so I don't take the chance.
Wanna calculate the cost of that new reef tank?? Take your worst nightmare, double it, and you're halfway there.