It dries fast, just a few minutes. I usually give it about a half hour or so. You can tell when it looks dry, it pretty much disappears.
If you don't have spares and the decals are more than a few years old, I would coat them. Another thing, I just load up small brush and make one brush stroke. I wouldn't try going back and forth with the brush, it dries too fast. Just a nice medium coat. Test it on a scrap decal first and you'll see what I'm trying to describe.
Bob