Googling about I'm seeing stuff saying that the demo was an hour to an hour and a half long, although that's preshow reports of how much time Nintendo were asking attendees to put aside for the demo. Here.
Taking the shortest of that estimate (1 hour), the turnover if every person played the demo fully comes to 140 people per hour. According to the E3 website the opening hours were 12-6 on Tues (6 hours), 10-6 on Wed (8 hours) and 10-5 on Thu (7 hours). That works out at 2,940 play sessions over the 3 days. With ~50k people attending the actual show, that's enough time for a little over 5% of attendees to play.
I think that was a separate in-depth press demo, that wasn't the same as the one on the show floor as I understand it. We had a hint of the limits on the show floor demo in the Treehouse when they forgot to turn off the time limit and had the demo end after... was it fifteen minutes or so?