The rule of thumb is to not put lacquer over enamel. Mainly because lacquer dries so fast. Where as enamel can take much longer. So enamel can still be curing, and when you put lacquer over top of it, it can go crazy. This happens as a reaction of traped solvents! Now how long it takes for the enamel to cure (not dry! Just because you can pick it up, does not mean it's cured!) depends on several things. Heat, air movement, humidity, and the temperature. So if it's cold out, it's not going to cure fast! I know the enamel's say about 7 days to cure, but it can be a very sensitive area to gauge.
The best thing to do is spray out a test panel. Let it cure, than you can spray the clear over top, and see what happens. Testors spray can paint could be some of the best, or the worst paint to work with. Solid colors usually layed out awesome. Where as their metallic colors, not so much! Test it first!