Yeah, when they aren't working any more. A lot of people will try to clean them with Windex occasionally, which seems to prolong their life some. But if that doesn't work and they're streaking, it's time to replace them.Is there any test on when they should be changed?
I'm a little under 2 and a half years and just bought my first set of replacement blades. I bought 26" and 19" in hopes it will cover more of the windshield - unfortunately the next size up for our rear wipers(14" instead of the stock 12") is a half inch too long to accommodate and they don't make a 13" in-betweener.Two years still good