After you die, you have roughly 72 hours to cross over. After this, your light stops getting smaller and disappears entirely. Then you have to find someone who can make the light for you...or you can find a funeral home during the wake of someone who has recently died and you can use their light. It's almost always best to cross over right after you pass though, because you can't realy accomplish anything here unless you make contact with someone who is sensitive and is willing to speak with both you and whoever you are trying to contact.

Eh, but what do I know? :P