Sobriety http://www.newadvent.org/summa/3149.htm
Drunkenness http://www.newadvent.org/summa/3150.htm
Two tracts that you should read to get full picture but for your particular question i.e at what point drinking becomes sin:
The sin of drunkenness, as stated in the foregoing Article, consists in the immoderate use and concupiscence of wine. Now this may happen to a man in three ways.
First, so that he knows not the drink to be immoderate and intoxicating: and then drunkenness may be without sin, as stated above (Article 1).
Secondly, so that he perceives the drink to be immoderate, but without knowing it to be intoxicating, and then drunkenness may involve a venial sin.
Thirdly, it may happen that a man is well aware that the drink is immoderate and intoxicating, and yet he would rather be drunk than abstain from drink. Such a man is a drunkard properly speaking, because morals take their species not from things that occur accidentally and beside the intention, but from that which is directly intended. On this way drunkenness is a mortal sin, because then a man willingly and knowingly deprives himself of the use of reason, whereby he performs virtuous deeds and avoids sin, and thus he sins mortally by running the risk of falling into sin.
For Ambrose says (De Patriarch. [De Abraham i.]): "We learn that we should shun drunkenness, which prevents us from avoiding grievous sins. For the things we avoid when sober, we unknowingly commit through drunkenness." Therefore drunkenness, properly speaking, is a mortal sin.
So, when you a)know that you drank too much b)know that what you drink is strong, but you abstain from it you still sin, not by drunkenness but by intemperance, venialy.
It is written (Sirach 31:32): "Wine taken with sobriety is equal life to men; if thou drink it moderately, thou shalt be sober."