I used to drink quite a bit on longhaul flights, but discovered that due to the reasons stated by sunnyjohn I'd get one hell of a hangover. Most times service continued for as much as I wanted, but then I'm quiet and fairly harmless even when drunk.
Once on an Iberia flight from Madrid to Montevideo I was told that 2 beers was the limit. On a KLM flight when I asked for my first beer was checked to see whether I was with a roudy group of people nearby who had their bar stopped. Eastern Airways limit you to two drinks on their flights as well, but then most flights are less then an hour.