But I wouldn't put it under 24 hour light. That really messes with them, they need at least 4-6 hours most of the time of dark. It'll put a lot of strain on the plant. I mean you can do it, I just think that esp. with a photo plant you need at least a minimal amount of darkness to make the transition easier and to give the plant a little time to stop pumping out chlorophyl and have a beer, go see the kids, w/e. As for how tall a lot of it(well most of it) will depend on the strain. A Sativa would go through most ceilings given enough time, where as indicas are gonna stop growing upward around ~4-6 feet in veg for hybrids and ~3-4 in heavy indica. Unless you're planning on making it a mother, vegging it for a few extra weeks trying to get another few inches is also giving it that much more time to develop problems, plus you're still feeding it, and a lot of those times the biggest thing you'll notice is space between where the buds are developing.