Totally agree. I was intentionally trying to make the most optimistic estimate possible for arguments sake. I agree with the Trader Sam's comparison, but I would temper it with Cove Bar, pre-expansion.15 minutes per person spent in the Cantina is not just really optimistic, it's just impossible. @Curious Constance could get one of those slurpee things at a 7-11 she raves about in under 15 minutes, but the Cantina is a different animal.
Using Trader Sam's and their service and business model as the closest current example, it would seem that 30 minutes per person-per drink is the most optimistic you could get. Although most customers would be closer to an hour and two drinks per visit.
Although, if I had to book a month in advance I'm going to make this count and have two drinks and an appetizer and I would expect to be there for an hour taking it all in, chatting with the bartender, pouring over the menu, watching Rex do his show and play his songs, seeing what happens to the machinery when someone orders the big fancy drink, etc., etc.
With that, there are two scenarios Disney could go with; the super efficient 30 minutes per visit ("Sir, you have to leave now") or the less structured 60 minutes per visit.
Super Efficient One-Drink & No-Appetizer 30 Minute Visit = 200 customers per hour
14 Hour Operating Day = 2,800 customers per day
More Relaxed Two-Drink & One-Appetizer 60 Minute Visit = 100 customers per hour
14 Hour Operating Day = 1,400 customers per day
The reality, especially for the first year when all the fans and AP's will want to linger and explore, is somewhere in between those two numbers. Either scenario is terrifying from a crowd control and customer service perspective. God help 'em, it's going to be messy!
All that said, for the opening night I will be very upset if they don't invite "the bearded guy" and get this fabulous WDW CM to recreate her schtick with a Star Wars theme. Now that he had his first drink a few years ago, the bearded guy needs to experience the Cantina!
Now if we could just get them to expand cocktail service to Tropical Hideaway and offer an AP special (look over here--bright shiny thing! Ooohhhh!)