how long before I launch into the main plot?

I have a story idea where the two protagonists are a ghost and a guy who sees ghosts. The plot is supposed to be the living guy developing locked-in syndrome due to a car accident, and the ghost helping him cope. (The ghost can read his mind.)

My question is: how much time do I spend developing their friendship? Could I just open the story at the car crash and develop it in flashbacks, or should I start the story awhile before the crash so you can see how they got so close?

I'd vote for opening at the crash. Modern readers are impatient; give 'em action from the drop.

Obviously both can work, but I like the flashback idea better, mainly because it sounds as a more "natural" way to enstablish their friendship, especially if it develops during a long time, which seems more probable. It'd save you the trouble of using time skips that make it obvious that that part of the story serves only for character development.

Personally, I prefer it when authors launch straight into the plot and build up or demonstrate character releationships during the course of the story. So yeah, crash followed by flashbacks but that's just my opinion.

Pretty much what everyone else said - start at crash, follow with flashbacks.

Of course, what kind of narration you're using could also affect exactly how you start. If you're going third-person, you could show the crash and give a bystander's reactions to it. If you go with first-person and tell from the perspective of the living guy, you could open after the crash, just as he hears or realizes or whatever that he has LIS.