La Leche League's THE WOMANLY ART OF BREASTFEEDING (p. 328) says:
The effects of alcohol on the breastfeeding baby are directly related to the amount the mother ingests. When the breastfeeding mother drinks occasionally or limits her consumption to one drink or less per day, the amount of alcohol her baby recieves has not been proven to be harmful.
La Leche League's THE BREASTFEEDING ANSWER BOOK (pp. 597-598) says:
Alcohol passes freely into mother's milk and has been found to peak about 30 to 60 minutes after consumption, 60 to 90 minutes when taken with food. Alcohol also freely passes out of a mother's milk and her system. It takes a 120 pound woman about two to three hours to eliminate from her body the alcohol in one serving of beer or wine...the more alcohol that is consumed, the longer it takes for it to be eliminated. It takes up to 13 hours for a 120 pound woman to eliminate the alcohol from one high-alcohol drink. The effects of alcohol on the breastfeeding baby are directly related to the amount the mother consumes.