One hint, make sure you shake the can really well. If you don't there will not be a good mix of the contents and you will get more of a matte finish instead of flat.
Its always much easier to add another coat than try to remove a coat that was to thick.
When to add another coat depends on thickness of just applied coat, temperature, and humidity. For a very light misted coat I would wait at least two hours. It may feel dry to the touch but underneath it may still be wet. You will know if you didn't wait long enough if the second coat goes all "wrinkly" looking. That is when you let it sit overnight and break out the fine grit sandpaper.