As an occassional contributor, I'll chime in. I like the idea of vehicles and I'm all set to participate.
That being said, I think this can be a one month project. I just started making vehicles in the past few weeks and once I got over the initial fear of it, it's actually pretty easy. If your goal is to scratch build a ship from styrene then you will need 2 months (at least). But if you are building off of an existing base, they are actually pretty quick.
My cargo shuttle took about 6 hours (paint time inlcuded). The Darth Vader Jedi Starfighter repaint was under an hour. My Battle Damaged Jedi Starfighter was 4 hours. The Clone Commnad Chariot took about 8 hours (3 of which were spent disassembling and cleaning the gross Mauler tank I got as a base -- it was rusty and muddy on the inside).
If people aren't comfortable with one month, then go with two. I just figured since most people here haven't started tackling vehicles yet, it might be good to give a perspective from someone who's done a couple.