I'm sure it's in the rules, or somewhere around here, I just cant seem to find it. What are the circumstances that a Player Character would turn into an NPC, and how long does that process take?

I just came into possession of a Lost Soul and I'm writing his back story. Just needed some hard data so I can fill in the blanks as to when he was reanimated into a zombie. Thanks for any and all assistance in this matter. Cheers!

The only time a PC becomes an NPC is if they drop their character.

Excellent! Thanks, Mikel.

I believe that if a player is arrested and still so after 3 weeks, they get an option to drop their character and respawn. This is slightly different from just using the drop feature as it gives ownership of the NPC to the arresting player (or maybe cuff owner?) and that NPC might be a Custom class NPC unless they changed that to Lost Soul as well. However this might of changed since the drop anytime function was added, or no-one bothers with it as they can drop sooner.