the air-speed velocity of an unladen swallow

...or a piece of string. That is to say: I need a formula for the unformulatable.

Given a warm-ish (please, weather gods) day and the unrestricted supply of alcohol, plus the distraction of some outdoor games and the occasional break for a photograph how much wine and beer can approximately 80 adults consume over a period of two hours?

I could think of this as:

D/12 (2 hours being one twelfth of a day, natch) + A – (G x px) x 80

I mean. I could think of it like that. It was fun to write it down. But it's not going to help me when I'm staring at crates of Peroni in Majestic. Can anyone help?