How many hours do I wait to breastfeed if I drink 12oz beer?

Zero. That small of an amount will likely not have any consequences. In general if my patients want to drink a small amount i recommend breastfeeding immediately so that the milk is used prior to the majority of absorption. Then the alcohol that is absorbed has time to be processed by the liver prior to the next feed.
Life is confusing. The effect of high dose exposure is well known and worrisome.Data on the low level exposure of infants to alcohol is difficult to come by because of problems with study design and population variables. We know th brain of a baby is changing rapidly in the first years.We also know alcohol breaks down to formaldehyde at the cellular level, which harms cells. I can never recommend avoidable exposure.