Impressed you use paper sketch, most mecha artists dont. With all the detail and such, but the end result is great, I forgot to comment on it when you posted it, but I love the feel of size you give it, and it seems pretty believable design.
The steps between each other would most likely take me about an hour at the start but from picture 3 onwards about 3 hours and then the final optimization probably wouldn't happen because I'm so lazy. It's great to have a little insight on how you produce these mind boggeling piece's though. haha