This.

When I feel I've payed too much, or when there's only wine, champagne, or other similar stuff left, can't stand those.

when i cant remember the date and time its time for a break :\

I can't drink alcohol, except for when I want my face covered with strange reddish patches and my head spinning, lol XD

When I had my third beer. Not that I am drunk after the third one, but I could get on a drinking spree and the hangover is simply not worth it.

Probably after 2 to 3 drinks.
I only drink alcopops really, so I don't get very drunk, and I normally burn it off after a few hours.
Sometimes I might have a couple of shots, (if they are there), and I'll drink a little cider if 'm offered it.
Not really much of a drinker though, so I get drunk pretty quickly, (but I'm a merry drunk, that makes jokes about everything, so I don't really get into any trouble).

Ask Mickey Rourke.
When I start to get dizzy, naturally... You know it's all good when your friend is forcing you to drink Martini from the bottle.

QFT
Tested yesterday, went out with my friends for a few beers on a cliff near the sea, and after the third we all got that "let's go to the shop and get more beers", and we knew, if we did, we wouldn't stop there. Thank gawd i'm broke.;P

I generally know how much I can drink over a certain period of time (around 5 beers over a couple of hours for example) to feel buzzed but not drunk and not get a hangover. If I'm drinking something that's not my usual (be it some unknown [to me] brand of beer, wine, etc.) then it's the mark where I start talking faster and faster.
Another easier way to tell that I should stop is if I am unable to mix one of the common drinks or don't remember some simple cocktail recipe (yes, I mostly drink with girls).

I'm not sure how others should know when it's time to stop . . . but I do know this from my own experience . . . =)
If you are commenting on how good the smooth, cool linoleum feels on your back . . . it's too late.
When you find yourself driving the porcelain bus, going to EUUUROPE to see RAAALPH . . . it's way, way. . . too late
Edit: Also, if you find yourself out and about wearing nothing but combat boots . . . it MAY be too late . . .

this
but I drink :p

Joining the "I don't drink" group.
1)I'm not particularly fond of the taste of alcoholic beverages.
2)I'm even less fond of the massive heartburns I get after drinking ANYTHING stronger than standard, 3.5-4% beer, even a tiny amount of champagne at New Year.

i don't drink alcohol

In my case it's pretty easy : when I'm a party, I know I have to stop when I begin to talk to people I don't know (otherwise I won't, I'm not the talkative/socially at ease kind). It means that I drank the right amount of stuff and that I feel good. Not a heavy drinker though, but that's what I'm looking for, I'm not trying to get completely wasted because I hate when people drink too much (and even more when it make them violent/boring).