Wrapping Up
Rule: You can't leave if it's your round.
Matthew, Mark, Luke, Lisa and Eve have been in a round all night. Luke has had enough and decides to depart, but it's his round next. Sorry mate, you can't leave just yet, it's your round. This much should be obvious.
But what if it's Lisa's round next, and then Luke's – can he leave then? Yes. You are only obliged to stay for the next round if that round is yours.
Of course sometimes you may have a legitimate excuse to leave (get back to work, get home for dinner, daughter's flute solo in school musical etc.). In these instances you need to buy out of the round - that is, buy a round for everyone except yourself (a type of negative-wedge - see The Wedge).
The Short Stay
Matthew, Mark, Lisa, Eve and Mary are at the pub. John arrives, but can only stay for two beers. What is he to do? If he buys the first round (of six drinks) but then needs to wrap up after only one more, this is clearly bad value for him. If he stuck around for, say, four beers and didn't buy a round, then this is a burden on the rest of the drinkers in the round.
The rule here is actually quite simple. If you will be in the round for 50% or more of the number of drinks in the round, then you are obliged to join the round, otherwise you do not need to.
The Pass-out
If you reach that special time in an evening where you can no longer stand up or keep your eyes open, then you are instantly excused from the round. Well done. However if you have a quick nap in a local park and experience a second coming, then upon your return to the bar you must buy back into the round.