Pumpkins here are left on the vines until the vines die. Where they are grown commercially, (not in Florida) they let the first few frosty nights kill the vines, leave the pumpkins sitting in the fields and harvest them all in October, in time for Hallowe'en and Thanksgiving.
I think a decorative pumpkin can be harvested as soon as it looks as pretty as you want it to. But the longer you leave it on the vine, the more likely you will have viable seeds to harvest.
