My guess is that its based off of the Endurance Talent. The higher your characters Talent, the more alcohol they can withstand. Also, if you eat more than one meal in a day your characters can become "Stuffed" which helps prevent passing out from alcohol consumption. Now, I'm not sure EXACTLY how the alcohol system works, in the attached photo my Rosalotta has a Tolerance of "190" based on her endurance stat, but I've never had a character last drinking more than 1 Wet Maria and 1 Hrd Dck in a session, both of which have an alcohol level of 50. The Journal page for Intoxicated also isn't very helpful, unfortunately.