Coach or sleeper, on the Auto Train you pick your dinner time at check-in. Typically, if you haven't arrived by 2:00 - 2:30 you can expect the most popular time to be sold out in coach.

I can't tell you what the most popular time will be, since that varies depending on how many passengers are traveling. If the train is very full, then dinner times will be 5:00, 6:30, and 8:00. If the train isn't especially full, then times will be 5:30 & 7:30. Typically with 3 seatings the 6:30 will sell out first, followed by the 5:00 pm seating. With two seatings, obviously the 5:30 goes first.

Ps. If indeed you are a first time traveler on the Auto Train, you might wish to check out this wonderful hints & tips report from On Track On Line. It will give you a good idea of what to expect and see on your trip.