There are all sorts of laminate sails. Some which will last only about 100 tacks. But lets focus on cruising strength spectra triradials.

We get about 25,000 miles from our sails. They are not shot at that point, and could probably go another 5000miles from there, but we we are concerned about the fabric integrity enough not to want to cross an ocean depending on them.

Three places get weak:

1. On the leach, right under the reef points where they get crushed by the reef line. You can minimize this a lot by pulling the fold of cloth neatly out before cranking the final few inches on the reef line.
2. For about 6" back on the leach, there is both fluttering and hinging that is bad for the Mylar. If you get an easy to adjust leach cord it will help - we get a 3:1 tackle in our cord so beth can tighten it in strong winds. And there is a lot of sailing making skill in spreading out the hinging action, preventing it from being being concentrated in one line and destroying the sail.
3. Chafe, on spreaders and lazy jacks and halyards. You need really good spreader patches, much more durable that the typical sailmaker patches - we put circles of thin UHMW plastic under bigger circles of spectra cloth under Dacron sticky cloth and sew the whole thing. For lazy jacks and halyards you just have to watch like an eagle and minimize contact.

The sail shape does change over time on these laminate sails, but in a quite different way that on a Dacron sail. The Dacron sail tends to just get baggy. On the triradial laminates the leach goes floppy and you get strange puckers at panel joining lines, and as I understand it there is some Mylar shrinkage due to UV. The laminate sails have much better shape at 4 years than a Dacron sail, but the shape has definitely degraded.