I'll chime in. I believe that applying a good UV vinyl protectant like 303 will increase how long your cover lasts. I have a bottle of 303, but am often guilty of not taking the time to apply it until I SEE the vinyl looking a bit dry... my bad! If you can apply vinyl protectant 2–3 times a year, that's best. If not, at least once per year (if you forget like me!).
Threads in the vinyl, especially at the center fold area... Once those threads start to deteriorate, your cover is on borrowed time. Maybe make a point to just do a quick spray on the center fold area a couple of times per year. Actually, I'm trying to convince myself here. LOL