I have once seen a flight attendant cut off a passenger for intoxication (in Coach even! But it was on the way to Vegas). However, there is a period prior to being completely outwardly intoxicated where they are "talkative". So they may not be beligerant and a general problem to the rest of the cabin, but they are an annoyance to the person sitting next to them. I sat next to a guy recently who drank maybe 6 (I'm estimating) doubles of whiskey/Coke during a flight from DFW-SAN which is a bit over two hours of actual in-flight time. If his drink didn't taste strong enough, he would ring the call button and request a top-up. While he wasn't outwardly intoxicated, he was certainly an annoyance to me. He actually physically took my headphone out at one point when I was trying to ignore his repeated attempts at chatting. And it's not even just insisting on having a conversation; when one is a little drunk, they have a greater tendency to talk about inappropriate things! I don't need to hear about your divorce and what a jerk your ex is and I certainly don't need to hear about the strip clubs you prefer in San Diego. I'm a girl. (Sorry, minor vent there!)

Either way, I would support an upper limit based on the flight time. While it is true that everyone can safely consume varying amounts, limiting everyone to a reasonable amount based on flight time would likely alleviate some problems.