Any time when it's appropriate to have a beer or to
start drinking. If you feel inclined to ask the question "
Is it beer o'clock yet?" the answer is almost always "Yes!" Thus it's usually a very
subjective measure.
I did once work in a restaurant with a semi-official beer o'clock, for those of us in the kitchen (around 9 p.m.). Having more than the occasional early beer was frowned upon by management and co-workers. Beer o'clock was announced throughout the kitchen every night and was universally observed. Every now and then a manager would declare an early beer o'clock and treat us to shots of tequila. Very good for employee morale.
If I were in charge of creating the metric time system, I would make "beer o'clock" one of the 10 official hours.