I haven't had to wash mine, I think they don't really get very dirty and so I don't see a need ...yet...
I made mine from a heavy muslin. I think I would consider replacing them if they began to be unstable (i.e. stretched in places) or if they seemed to shift. I use Leadergrips with mine as well vs pinning, so maybe if you pin that might make a difference? I can't say, just posing it as a thought that may affect wear...