We have had some sheets wear out at the ratchet block close hauled position-- that is about ten feet from the end. Not much wear else other places. End for end the sheet and wear the other end a little at both ends and it should double the life of the sheet.
I would guess it will take 2 to 4 years to wear one out depending on the block, how much wind you sail in, how long you sail and how strong you are.
Suggest end for end each Spring and maybe Anna will do it every regatta when she gets back to sailing Lasers.