A) ASAP. Hopefully before the next run. Gotta track every mile.
B) PDQ. You know the distances where you normally run though, so you can still track your miles that way.
C) Someday, but you want to shop around for the best watch.
D) Someday, but you want to shop around for the least expensive watch with the bare minimum functionality that you need.
E) Never. Maybe you don't even have one.
I'll admit that my answer would be B or D. They got me hooked. I would be pretty quick to replace my GPS watch.