Usually about 2pm. And this is the norm for nearly all of my friends and neighbors, which makes me wonder if there is a regional variation to when Thanksgiving dinner is consumed.
(The fact that it's called "dinner" to begin with always seemed weird to me for this reason since I've never observed it being eaten in the evening anywhere other than in Thanksgiving TV sitcom specials.)
For me "dinner" is the big meal of the day, whether it is served at lunch time or supper time.
Our family typically aims for 1pm with the understanding that a turkey that takes longer or shorter than expected to cook will alter the start time.