That's a good question... I always thought he died and the moon brought him back to life by turning him into Jack Frost.

Also, just how long was he under the ice before he turned?

Well, before he fell into the water, he was living in a log cabin in the middle of some forest and he and his sister were wearing some clothes out of leather and home-spun wool I think. That could have been really at almost any point in history if someone else has more info please comment. When he wakes up he goes to a town which could have been in the USA when it was first being colonized.

I think he died since the Tooth fairy tells him that each one of them had a past life before he becomes a guardian. When he fell on the pond the moon gave him his abilities since he saved her sister from drowning.