It's two hours long. From my cable TV box's guide:

"Commemorating the 60th anniversary of what is long considered Walt Disney's crowning motion picture masterpiece, "Mary Poppins"; featuring rarely seen footage, photos and stories from the film's living legends."

(And ABC is airing Mary Poppins on Thanksgiving Day, from 8:00 PM to 11:00 PM.)

A bit late, but here's a preview that aired during Good Morning America:



Floyd Norman's memories of Mary Poppins:



Props and a storyboard from Mary Poppins that are in the Walt Disney Archives:



And it looks like this documentary is going to be available on Disney+ and on Hulu.

This special was produced by Richard's son and nephew (whom also did the Sherman Brothers documentary, "The Boys").