All I can say is last year bought a cheapo 8 mil and found it was too flimsy to roll easy. Got a 12 mil for $106 at Leslies and seems fine. Exposing the rolled cover to sun is a real killer. The white plastic sheet they give you is barely enough to cover so I got some cheap white bedsheets at Walmart and stitched them together. Works a lot better. Best you can expect is probably 3 years and anything after that is gravy.
My nickels worth.
Al