There are a lot of types and kinds of paysites

A site comprised of licensed content from the usual sources, non-exclusive or even exclusive, may grow tired quickly because customers 1) recognize the images/videos from other sites, 2) recognize the actors/models from other sites and other works, or 3) recognize the shooter's sets, backgrounds, locations or style.

"Oh, that's so and so's work from Prague."

A site built upon a Star or Point Of View theme will last as long as the supporting actors/models provide new interest. Like Deanb's site - Dean being the thematic device and "his boys" the fresh incentives, or Rob's Military Classified where he has a new guy to blow or get topped by each week...

Can't give one answer to fit all paysites, because there's so many types of paysites.