Main showtimes in the theater are 7:30pm and 9:30pm - mostly. Could be shifted 1/2 hr later on Euro itineraries. Some headliner shows may be set 15 min off the preceding. In addition, there are other entertainment experiences scheduled at various times - but a ship will often have a discernable pattern to these once you see a couple of schedules on board (formerly the Princess Patter).
So, figure out how much time you want for dinner (at least budget an hour), plus how much ahead of time you want to be in the theater, and book accordingly. Or, do walk-up if you prefer. If you book and are totally wrong for what you want to do, then you can work with DR Hosts and make changes.