I played a bartender for about five months straight, barely leaving from behind the bar.
It's handled per real-life week, the status update coming on Monday afternoons US Eastern for me. I don't know the exact formula, but whether or not you level up or drop seems to depend on both the number of drinks served and the amount of time spent behind the bar ("behind the bar" being time with your bartender rank visible). As you level up, you gain title changes, and drinks cost less to pour (meaning more profit for the bartender). Bartender levels are also specific per bar, so a Nomad Brewmaster would start as a Cloven Hoof Server.
I did manage to maintain Brewmaster status for most of my bartending era (thank you to everyone who bought "Nellas"'s weight in kaeth twice a week) but I also did almost nothing else with my Arelith time. Even with that, I remember losing a level around Christmas/New Year's due to the reduced flow of players through the area buying drinks. I agree that the requirements feel extremely steep at the moment, and some more leeway could be built into the system.
Rolled: Solveigh Arnimayne, "Anna Locksley"
Shelved: Ninim Elario, Maethiel Tyireale'ala
Current: Ynge Redbeard, ???