I don't know if a precise formula like that exists but the second layer does have less firm foundation than the first and it would be logical to expect shorter life span. Still, you've got no less than 17 years from it already. And shingles have started to separate. I think you've got a good life out of it. It's sounds like a great time to strip it all down to the decking, remove old flashing, check for signs of rot, repair what needs to be repaired, put new flashing, felt and redo the shingles anew. If it showed no signs of damage, I might have left it alone for a few years longer but since it's already started going, you should be looking at a possibility of a tear-off sometime soon.