Mine seem to go in bursts, like I'll check 'em after 24 hours and it seems like the buds are way bigger, then they'll go 3-4 days without seemingly changing at all. If you put another light on them when your existing lights are on, I don't see a problem. Just don't put it on them while the plants think it's supposed to be dark. Whether it will do any good or not depends on what kind of light and how close you can get it. Are you feeding with a bloom fert of some kind?