My wife did a ton of searching around for answers on this. The general rule of thumb that she found was that breast milk concentration goes as blood-alcohol concentration does. We fully metabolize about an ounce of alcohol in 3 hours or so. The best advice I've seen is to pump directly before drinking so you can store the un-contaminated breast milk.
This link from La Leche League says the following:
Thomas W. Hale, R.Ph. Ph.D., member of the LLLI Health Advisory Council, says this in his book Medications and Mothers' Milk (12th ed.):
In an interesting study of the effect of alcohol on milk ingestion by
infants, the rate of milk consumption by infants during the 4 hours
immediately after exposure to alcohol (0.3 g/kg) in 12 mothers was
significantly less (7). Compensatory increases in intake were then
observed during the 8 - 16 hours after exposure when mothers refrained
To answer your question
How much alcohol is transmitted through breastmilk?
The above link also states:
In a study of twelve breastfeeding mothers who ingested 0.3 g/kg of ethanol in orange juice (equivalent to 1 can of beer for the average-sized woman), the mean maximum concentration of ethanol in milk was 320 mg/L (5).
This publication from the National Institute on Alcohol Abuse and Alcoholism states:
When a lactating woman consumes alcohol, some of that alcohol is transferred into the milk. In general, less than 2 percent of the alcohol dose consumed by the mother reaches her milk and blood.
Both sites show that alcohol isn't stored in the breast milk. The body will metabolize the alcohol, both from your blood stream and your breast milk over the course of a few hours (depending on how much you've had to drink).
That means the alcohol level in breast milk will reduce at the same time it does in your blood, so you don't have to worry about alcohol in your milk hours later (so long as you don't drink enough for your blood to still have alcohol in it hours later). You may want to wait to pump or feed again until it's had time to metabolize, but you don't need to throw the milk away.