How long should you try? Until.
When confronted with iron-will determination, I can see Time, Fate and Circumstance calling a hasty conference and deciding, We might as well let him have his dream. He's said he's going to get there or die trying.
Confronting and overcoming challenges is an exhilarating experience. It does something to feed the soul and the mind. It makes you more than you were before. It strengthens the mental muscles and enables you to become better prepared for the next challenge.