That's going to depend on how much weather it sees. Heat from the sun, Winter cold, wind, rain, all will have an input on longevity. Under normal daily usage I've seen them start to break down in just 2-3 years. However, seldom used, doesn't sit in the sun, never sees snow, they can last years longer. Another factor is whether it stays in place or gets removed/folded and then re-mounted on a semi-regular basis. Garage kept or parked outside can make a significant difference.
I've never been a fan of vinyl tops, they just don't hold up, especially compared to canvas. I have actually seen original WWII and Korea issue canvas that was still servicable. Vinyl, on the other hand, seldom makes 10 years, in my experience. Others may have had better results.
Keeping and storing a vinyl top presents it's own problems. Cool dampness will lead to mildew, sometimes severe. Too warm and the plasticides start to depart. Long term storage of a vinyl top can lead to the creases "setting". If it's sat long enough and the creases set hard enough they can actually crack when finally unfolded.
OK, after all the gloom and doom, in my opinion, I would sell the extra. If you need a new top down the line, buy a fresh one.
Larry
