The question is - 'When to quit' ?

[VitoT]

We all know that better to quit early when you are losing, and continue to play longer while you are winning.
All the succesfull players talking about this like the best recipe for maximizing your profits.
for example : many gives themselfs a limit of 3 buy-in lose on 1 session. but when u decide to quit while you are winning ?
many times I find my self up like 3-4 buy-ins and just can't keep winning, and ending the session up only 3 buy-ins or maybe even less.
In the other way, i see a winning streak by players who gets up like 10-12 buy-ins in a session, and it seems nothing can't stop them that time, even if they are playing bad.
But me, when i'm up many times like 3-4 buy-ins, early in the session, and all seems like going my way, i just start having bad beats and it becomes very frustrating.
So where are your limts, and how you see it ?