It usually takes about a half an hour before alcohol gets into your blood, and milk. So if you really wanted too then you could breastfeed while or just after drinking and you should be fine. Although it is advised not to drink at all while breastfeeding.
Anything that you drink or eat can be passed to your child through your breast milk. Thus, it is best if you avoid alcohol entirely while breastfeeding.
If you will be feeding your baby within a short period of time, it's best to skip it entirely or have only one small drink. Alcohol does pass into breast milk, and while it will clear itself out over time, this will take many hours if you have had several drinks.