I'm not an expert with car paint, but I have a lot of experience with commercial paints (and coatings) for buildings, so FYIW, additional layers of coatings bond with the solids of previous coats. Drying, (the off-gassing) of the delivery vehicle (the paint base, or solvent) can continue as long as the new coat is porous to the solvent of the first coats. So in theory, as long as the solid part of the initial coat is clean, additional coats can be added at any time, up to an indefinite time after. I'm not sure if this is applicable to automobile paint, but it seems reasonable that it would be. If I'm wrong, I'm confident I'll be corrected here.