So did some math. Its probably safe to guess that for the sake of time, Best Buy matches will be two minute, 4-player FFA's. Last year the Best Buy I went to had a decent amount of people, definitely around the 100-person mark, however there was only one set-up. I'm sure there were other areas with much more people, but lets say your local Best Buy has (somehow) only 100 people in attendance. Each match will take at least 2 minutes on it's own, character and stage selection time, as well as potential sudden death matches, so each round will be around 2.5 minutes at best maybe 4 minutes at worst. With 100 people there will be 25 groups of 4. This means from the end of the line, it will take 62.5 - 100 minutes to play Smash Bros.
This is also only if they don't announce any additional games for Best Buy demoes (In fact I'm willing to bet Mario Kart will be playable there as well), which probably wont have a hard time limit like Smash will have. And as I said, plenty of places will have way more than 100 people, meaning if you wanna play again you will probably be waiting two hours at least, at an event that last year only lasted four. In other words, they better have more than just the one set-up this time.