Oh, I'm not saying that good art doesn't take lots of time! My general impression from my own fiddlings, talking to friends who are artists or writers, and just reading about what the pros say, is that most of the major steps take roughly the same time, give or take the natural variations in types of stories and individual people's workings.

Correct me if I'm wrong, but for most manga the writer and penciller are the same person, right? Even with a big team of assistants I've always wondered how mangaka manage to both script and write a twenty-odd page installment every week without, you know, imploding.