I’ve got plenty in stock of the clear that I can do by the metre for you. I usually replace mine once a season unless for some reason it lifted or some damage had occurred that means removing it to do the repair.
All advice I have seen is that a sheet should be changed at the end of every season although I must confess I have no idea why.I believe the sheet does keep moisture in, but you still need to oil the exposed edges and back to to retain the moisture in the bat