They open at 7 Am Eastern time. If you want a popular restaurant, the popular rule of thought is to be DIALING at a minute or so before 7 so that, once it hits 7, you're already through the phone shpiel.
Not sure when they close, though I'm thinking 10 PM.
ETA: Yup, 10 PM, though they close at 8 PM on Christmas and New Year's Eve per Deb's website.