Working on straightening a bowed fiberglass fender, I jacked out the center to remove the bow and added a rib for strength. How long does in have to cure to ensure it stays in the correct position once I remove the jack?

IMO, the resin may set within 30 minutes, but if you are trying to solve a warpage issue, I would keep it restricted at least for 24 hrs, and longer if possible if real important, say 48hrs, a lot also depends on your ambient temps, I would not expose to sunlight to accelerate the cure.

Good luck with that !!! Once fiberglas has taken its shape it is virtually impossible to correct. And the longer it sits the more stable it becomes. You could have set it under some heat lamps for a while before putting in your rib to soften it up, that "might" have helped. If the fender bow has more strength than the rib it will just bow back.