I think the very next point may shed a little more light.
2290-2291. The virtue of temperance disposes us to avoid every kind of excess: the abuse of food, alcohol, tobacco, or medicine. Those incur grave guilt who, by drunkenness or a love of speed, endanger their own and others' safety on the road, at sea, or in the air.
The use of drugs inflicts very grave damage on human health and life. Their use, except on strictly therapeutic grounds, is a grave offense. Clandestine production of and trafficking in drugs are scandalous practices. They constitute direct co-operation in evil, since they encourage people to practices gravely contrary to the moral law
Are you looking for an exact moment when drinking becomes sinful? I think it's probably left a little more to interpretation, but I would say that whenever someone becomes drunk they would endanger their own and others' safety. Alcohol is a drug--that is how it is classified...just like caffeine--and the sinful use of alcohol would inflict damage on health and life. I am not saying a drink or two here or there is that way, but I am answering in regards to sinful drinking.
I'm not a Religious so...I can't guarantee it's right, but thats what I would say...