RE: how much time?
Depends on the person--I'd say that most folks wouldn't be ready to hunt this season.
IMO, you need more than just back yard practice. I've seen folks that could wear out a target in their back yard, but (even with wheels and sights) not be able to hit much at all in a different setting. Stump shooting and 3-D is great practice, a confidence builder, lets you learn more about your limitations, and is a lot of fun to boot.
Chad