Can't say there is a "rule of thumb" answer to your question.  To me, the top was meant to be down!  Condition the plastic with one of the spray products out there.   I use Plexus.   Make sure the plastic is dry when applying.   I keep mine down for 1 week and then raise it for a day or two and apply the Plexus.   Mine shows wear right in the middle where it creases when folded.   Don't think you can do much about that over time.