Yeah, that's pretty ambitious, size-wise.
I can tell you from experience, big cities, done hi-res, take a lot of time.
I say that as I'm working on one now for over six months and still not close.
I like what your doing here. I think it will be worth it. Though after six months you might not agree.
Takes stamina to keep at these big city maps. Stay strong ascanius.

edit - It looks like you're right there with me time-wise.