Hey Charlie!
Like most paints/films, you probably want to let the stuff dry at least overnight to be safe, but I've been able to put successive coats on a model within 10-20 minutes of each other with no problems at all. However, I usually spray the stuff and I spray kinda dry. If you spray wet or if you're brushing it, the longer the wait the better.
Future doesn't "act" like many of the actual model paints, it's VERY forgiving. It's self-leveling which means that if you have some heavy wet spots or brush marks, it will usually settle itself out no problem. (Unless you've simply put WAY too much on or if you've brushed it on too dry.) It will sometimes turn a milky white during application, especially when it's put on over a previous coat that hasn't fully dried or if you simply try to put on too much at once. This milky look will go away.
By the way, what are you using it for? Gloss for decals? Or are you using it on a canopy? If you're using it on clear parts, I'd recommend that you wait at least a full day because any imperfections will be more visible on a canopy as opposed to if you were glossing the model for decals.
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