Alcohol appears in milk as soon as it appears in the blood stream.
Alcohol is freely diffusable back and forth between milk and blood and
as the blood level lowers, milk alcohol levels will also decrease as
the alcohol diffuses back into the blood.  After one drink, milk
alcohol levels will usually be undetectable within a couple of hours.
However, some people metabolize alcohol less rapidly than others, but
this is true of any drug.

I am 100% in disagreement with anyone who says a mother cannot drink
while breastfeeding and she must sacrifice breastfeeding if she has
even a single drink of alcohol.  I am particularly surprised that
lactation consultants feel this way.  This is the new puritanism.  It
is a message to mothers that breastfeeding interferes with normal
life.  Given that most of us believe that artificial baby milk may be
responsible for increased risk of diabetes, lymphoma, etc, etc, why
are we telling mothers to "pump and dump" (aaargh) after a drink or
even a night of drinking?  A mother too drunk to take care of her baby
is still better off nursing him than trying to prepare formula
(presumably dad has indulged as well)--why do we imagine she'll
prepare formula right?  At least the milk won't be too hot, or too
concentrated, if she breastfeeds even if her milk does contain
alcohol.  Remember, most jurisdictions consider you too drunk to drive
if you have 0.08% alcohol in your blood.  If that's what you have in
your blood, that's what you have in your milk.  That's nothing.  Even
dealcoholized beer is 0.6% alcohol which is 7-8 times more.

Jack Newman, MD, FRCPC