NCL doesn't have any set dining times... that is, there is no early or late seating. Instead, the main dining rooms are open from about 6:00pm-10:00pm (at least on our Med cruise), and you go whenever you want, or you can make a reservation. You can even request the same waiter, if you'd like. The main dining room menu changes every night, and is posted outside the dining rooms that morning.
Reservations for the extra-charge venues aren't necessary, but suggested, particularly the Teppanyaki Room (which was always filled).
A buffet dinner is also available (no extra charge) from 5:30-9:30pm.
There are TV screens in various places throughout the ship that give you an idea as to the wait time for all of the dining venues. On our cruise (which was a bit out of the ordinary as we were sailing at 75% capacity), there were no waits for the main dining rooms and several of the extra-fee dining rooms.