It depends on your travel time from restaurant to theater, and how many courses you plan to have. I usually eat at 6pm for an 8pm curtain if the restaurant is a short walk (less than 10 blocks) from the theater.
assuming your restaurant is near the theater, I would get there no later than 6 and let them know you are seeing a show. if you are paranoid like me, get there at 5:30. many restaurants are having earlybird specials so your choices are many!
Guess , viewing other replies, it depends on how 'conservative' you are with your time. I tend to figure 6:30 for a theater district restaurant is early enough. So you've got a full range of replies here, and probably not all that helpful!
i just went and it started a little after 8; all restaurants in the theatre district cater to people who are going to see plays...they are SO fast with your meals. You can be in and out in less than an hour. 5:30 in ENTIRELY too early. TRUST ME.