You will feel when its ready mate.
Once the first coat is on, clean your hawk, trowel, buckets etc... Then take the lines out of the first coat with your trowel, starting where you started laying it on.
Clean your trowel and get set up for the next mix.
When your mix is ready, take any large lines out that are still left, and get your second coat on.
When you are taking the lines out of the first coat, you will notice the plaster is stiffening up a bit. If its not, then wait a little longer and try again.
Basically, your second coat goes onto the first, when the first coat is 'pulling in'.
Don't try to lay your second coat on when the first is still wet, or it kinda defeats the purpose of a second coat, as the first coat will move around underneath your trowel.