Neither. It is a totally subjective call by the official. It's not until he deems that the non-offending team is in a position equal to or better than it would have been had the offense not occurred that he ends the advanage. In the case of a knock-on advantage, it might end when the non-offending team kicks the ball away to gain territory. But if you have a penalty -- particularly a penalty in a kickable position -- then he will probably let play continue until (1) the non-offending team scores or (2) the non-offending team loses possession or (3) there is some other natural break in play. In the last two cases he will then award the penalty. This means the advantage can go on for a pretty much unlimited amount of time.

It's up to the ref. Pretty well described above, so won't repeat, but I will add that an actual advantage has to have been gained, not just an opportunity. For example, your team has the advantage call, and you put a player into a gap with a great pass - he should score a try, but drops the pass. There was no advantage just because he had the opportunity, by dropping the ball there was no advantage.

It's up to the referee. If the advantage is for an infraction which would result in a scrum then it may only be a couple of phases or if they other team secures possession. If it's an advantage for a penalty then it can be even longer. If the other team commits an infraction or they aren't clearly benefiting (such as scoring or creating go-forward) from the advantage then they will go back. So in short, it's all up to the referee.