I have seen a couple of turorials around, although I've never used any so I couldn't tell you where they were.

I would say the best way to learn how it works, is to open a simple theme and read over it, see how it's set out. Then look up which modules that theme uses and flick through the documentation for them.

As for time, I think you need to make a decision, and I warn you now that choosing Litestep will waste you more time than you have comprehended so far, and it's adictive... That said, it's far better than Aston Shell...