When you know to move on.

When that person is completely ignoring you, move on.
When they have screwed you over terribly and its obvious they dont care, move on.
If there is a void in your life and they dont care if its filled, move on.
If there was never an intimate bond to start with, move on.

Fact: Obsession doesn't accomplish anything. It never will. Loosen your grip, let them go.
Fiction: That person will come back around someday if you just hang out and wait for them.

My advice to you is this... there are people in the world that just plain don't care at all about you and if you are going to sit around and mourn over them for eternity you wont get anywhere in life. Trust me, im trying to help and im trying to save you alot of time and heartbreak.