The time at which caffeine is required to commence or continue mental and/or social function. Most common towards the end of the week or during a night shift.
See also "half past caffeine"
Person 1: "You've been staring at that screen for an hour, and haven't done a thing. You know this work's due in an hour right?"
Person 2: "Yeah, I got like four hours sleep and can't concentrate. It's definitely caffeine o'clock."