Yeah, go by what Mike Phillips says :xyxthumbs His wife Lynn has posted some good stuff on using #7 also.
#7 is sorta weird in that it doesn't really "dry" the way most products do. And it's really not all that great on b/c paint, just not what it was designed for. #81 and #5 (and even #3) work better on b/c in my experience.
How long you let #7 dwell can depend on what you're trying to do. I've let it dwell overnight (thick application too) on older ss. But if you're using it on b/c, there's no real point in letting is dwell any longer than it takes for it to "skin" since it doesn't "soak into" the paint the way it can with some older types of ss.