When do you switch to a new pair of goggles?

If you have used the same pair of goggles for a long time (maybe even years), what eventually makes you dump it and get a new pair (could be the same model)? I have a pair that I expected to last at least a year, but the lenses have had so many scratches after just 3 months that I can't even see clearly. I can't think of how it got scratched except I always wiped it with tissues after swimming (it's kept in a plastic case). It's not dirt but clearly scratches. In the past I was done with a pair only when it started leaking. I'm curious if blurred lenses due to scratches are common? (The poll question is: "I use the same goggles that work for me for as long as I can until" --for some reason the "until" didn't appear)