When Should you give up?

Im sure everyone has had a few things in life that they wanted so badly but yet have not been granted, and deep down inside still yearn for. When that arises what do you? Be it a person that you want to want you or a job/company that you have been applying to for so long and have yet to get called for, or whatever else you can think of... when is it time to say goodbye and realize that maybe it wasnt meant to be?

My answer to this is... well...
I remember when I was little I used to say that God had a big library in his house and in this library were books of everyones lives. So whatever was happening to us was supposed to happen because God had written so in our personal books. (Biographies if you will) If I can revert back to this childish way of thinking, I would say that I would love nothing more than to take a sneak peak at my book and see what is in my future. I would only look for that which has eluded me for sometime now, Nothing more. I feel as thought it would give me peace of mind, so that I may know that it is only a matter of "when" instead of "if" and if so, "when". ( Are you following) . But because that will not happen one must keep the faith. If I was meant to have this then I will, if im supposed to work for the company of my dreams then it will happen...

So when is it time to give up? I say... as long as you have the energy to keep trying and striving for what you want...the time is never.