No hard and fast rule, Sugilun, but 12 days after roasting is probably getting towards the long end of the optimum time. Perhaps the roaster man was referring to a specific bean which likes to breathe longer than most others.
I try to leave at least 4 days, but thats often hard to do because Im always late roasting the next batch. Today, for instance, I am forced to use 2-day old beans which definitely need more rest.
Even milk can barely mask the acerbic taste.