The permanent wave was likely caused by either putting something hot on it, leaving it in a hot car or not storing it flat. They can be very temperamental, so you have to be careful with them. When I first started quilting, other quilters told me not to put my tea cup on them, or anything else hot and I've learned from experience not to leave mats in the car when it's hot and sunny outside. For storing them, if it's a small board, you can use a pants hanger and hang it in the closet. For my big board, my hubby put two holes in it and then two nails behind a door and it hangs there when I have to collapse down the table it usually goes on.
I hear that the white cutting mats are not as good as the others, that they will keep threads down in them. Keep that in mind when you find some funds to get a 2nd mat. And you can use coupons at JoAnn's for cutting mats. There are mat cleaners that you can use that are like a fine sandpaper.
Your original question of how long do they last depends on how you care for them, but I would imagine they could easily last 15-20 years.