I've gone clean a number of times. Last was when the wife was pregnant w/ twins. I stopped my drinking entirely at about 5-6 months pregnancy and was down to a single drink/day prior. Did not have a drop for over a year - till the kids were sleeping through the night. Had a couple beers watching the Beijing Olympics (Go Phelps!). Up to 3 drinks I don't notice any difference, same with coffee. At 4 and more I begin to feel it overall, but is really individual, with age and mass playing a huge part for most. Andre the Giant was an crazy party animal but probably seldom "drunk" - have you ever seen what a can of beer looked like in his hand?! - smaller than an 8oz can of V8 in mine.
When I was in my 20s I could do some superhuman drinking and lifting, but even then I always had a trigger point for bad hangovers going back to my late teens, self limiting and for that am grateful. Otherwise I might have been good example of Irish proverb "drink takes ...".