If you want to add a night rating to your PPL(H) national or JAA you need the following;
10 hours instrument flight training, 5 hours of this can be conducted in an approved simulator.
and 5 hours night instruction, including 3 hours dual, a navigation flight and 1 hour solo including 5 solo circuits.
So its 15 hours in total.
See JAR FCL 2 or LASORS for full details.