Congratulations, Jeff! That rules!
So, are you guys going to cut a promotional demo, or are you thinking more along the lines of a CD to market by yourselves - like at gigs and stuff?
First off, I'd say be ready for the fact that set-up, level check, and mix-down are going to eat up a good bit of those six hours, so set your goals accordingly. Go ahead and meet with whoever's going to produce the session, and discuss what you can realistically accomplish in the allotted time. What the heck - you may collectively decide that six hours is just a good running start - depending on your goals and your budget.
With all that decided, choose your strongest material and rehearse it until it's second nature. If you can give me some more of the details - either here or in a PM - I'll be glad to offer any help I can.
I'm stoked, Jeff! I think you'll find the studio environment is a gas!