As I remember previous Expos, most of the sessions are held from mid morning through late afternoon. I remember the Expo exhibit floor being open from from 8AM to 6PM.
According to the D23 Expo website, they are expanding the exhibit floor this year to include the Anaheim Convention Center's exhibit halls A, B, C & D. Previously they just had halls A through C, with D unused. Those four halls together equal about 675,000 square feet of space, and it's where they have the major division pavilions, the collectors forum, and all the various exhibits from the Disney empire. The archives exhibit is up on the third floor in a big ballroom, but otherwise all the "eye candy" stuff is on the exhibit hall floor.
As a point of WDW reference, the largest exhibit hall of WDW's convention facilities is the Veracruz Exhibit Hall at 86,000 square feet. The point?... Give yourself plenty of time to explore the D23 Expo exhibit hall and its 675,000 square feet of space.
(Although, personally, a bunch of the stuff on the exhibit floor I skip as I'm not interested in everything Disney; their wedding planning group, kindergarten learning toys, junior high bedroom sets, adorable Disney Channel "stars" etc., etc.)
