I'm always confused about these sort of posts.
They gave you 2 weeks, an idea of what they want from you, and an unlimited vert count and texture res budget.
You say you really want to work for them. If that's the case then you should know what style they would typically be looking for....so there's your inspiration. You should have played most, if not all, of their games in the past. So, load them up, take a look at the texture res and average vert usage, and be smart about what and how you create.
As far as the time is concerned...every spare moment you have. I know for me, this is my life, games/art...I've always poured everything into anything I'm tasked with; art tests and work alike.
You've already been contacted about how they enjoyed your work, so you're farther along than most. Just get to it and kick ass.
Side note; I've yet to see or hear of any insane art tests.
edit: Also, shame on your previous company for not allowing you to show the work you've done. Personally, I think if you left on good terms, you should always be allowed to show the work you did; maybe not publicly, but at least to potential employers.