How long should an aspiring actor stay in LA auditioning before they decide to go back to their prior life?

While I am not an expert in this area, I do have opinions, and they go in two directions. One is that I think it is important for each person to give himself enough time to satisfy his own sense that he tried for long enough--for some people it could be a year, for others it could be five years or longer. But whatever that amount of time is, and it might be stated out loud, when it is over and one has not had his career take off in the way he had hoped, then he can feel he has tried his best and then can move on to another (perhaps former or perhaps different) life. The sense of personally really trying though should not be cut short. And the time when it runs out for the experiment should be acknowledged.

Three years.

That's the average amount of time, I've heard, to get things going. But the most important thing is do be doing something other than acting so your head doesn't explode when your agent never calls you.

3-5 years considering you are auditioning as much as possible.

27 years

Give or take 30 years

stay if you're happy. leave if you hate it.

I'd say it's best not to think about the negative (quitting) and better to think about the positive (what are you going back to/onto).

Friends/relatives who are successful actors say that the only reason that they've been successful as compared to other similarly talented people is that they can handle daily rejection, not take it personally, and keep going. So as long as he can tolerate the daily rejection, he should keep going. He may never be a mega-star, but he'll get some work on occasion--law of averages says so. But when the rejection gets too hard to bounce back from, maybe then he should move on.