Mine have been washed for over one year on a regular basis and they are a little worn but still in excellent condition.
I just wash mine when they get noticeably dirty or wet. They get cuddlecups in their cage which gets clean every 2-3 days, so I check them then. If they aren't very wet or dirty then they stay a bit longer. Usually there are urine stains in the bottom after 2-3 days, so I wash around once a week.