This is the kind of thing that will definately be strain/plant specific, and even on approach since trimming the upper stuff pretty viciously can speed up the process, but with obvious drawbacks.. Then we kinda gotta decide when the proper label would be revegging, rather than reverting..The larger the plant, the longer it will take to revert.. When you do revert, expect some funny mutant unserated, single lobed foliage.. It will go back to normal eventually though..
So to summarize, any plants that can be revegged can be reverted at any time, you just need to decide what you consider feasible.. If you want to trim the hell out of it to speed things up, or wait a few weeks to a couple months maybe for a fuller plant plant to kick back into early season type growth.. And really, in my experience with reverting, even after only a few days into 12/12, the plant will be pretty dedicated towards flowering.. I've flipped them back at 4 days and had to wait about 3 weeks to really be happy, and I've flipped them back at 16 days and had to wait roughly the same amount of time.. But then again, those were two entirely different genetics.. NL5, and Skunk#1 respectively.. (Furthermore on that note, the NL5 blasted out pistils alot more quickly after flipping than the Skunk#1, but that makes sense, because the NL5 pheno's were much more indica prevalent..)
Edit: I don't know if autos can really be revegged.. Recently somebody who I think knew what they were doing, tried but the plants apparently died.. Sorry I can't remember who that was, or in which thread..