Scientifically, about 2 hours, per drink. So if you had a glass of wine at 9pm, the alcohol would be so minimal by 11pm as to not really register. A better rule of thumb is "if you can feel it, baby can too", so if you have that pleasant wine warmth, beer buzz or downright drunk going on, it'll be in your milk.
If you really want, you can get little dip sticks to test to see if you have alcohol in your breast milk.
Me personally, I do not pump and dump. I rarely drink never more then one at a time, and it never bothered Tiny when I did have a drink (in fact, the Yule she was not quite 2 months old she slept really well if Mommy had a Kahlua in hot chocolate in the evening). But that's how it affected MY kid. With my last 2 nurslings, I picked a time where if we had a sleepless night it was not a big deal, and had a beer (dark beer is good for milk production) or something and just timed to see how long I felt the effects (can change due to hormonal changes) and how it affected baby. Moose always got hiccups if I nursed within 3 hours. Tiny was never affected.
Let me know if you want my links, I don't have time to search in the morning, but I have information I can copy rom my textbooks for you!
Need to update a ticker, Silas Anthony Jordon born Oct 11, 2013. 6th baby, so much love!