Hi Rykinner, my reply doesn't bear any guarantee, for it is strictly based upon hearsay (from other coffee forums) and my own experience.
From coffee forums and blogs, I understand they keep at least one year.
This is also my personal experience.
I keep my green beans in one kilo batches, in jute bags, shut close with a piece of hemp string. The bags are stored on the other side of the wine-cellar wall, in a dark cupboard. I don't stack the bags one upon another, keep them pretty much separate.
The cellar is far from perfect, in winter it will be a cool 5-10 degrees (Centigrade), reaching upto 22 in summer. And it's far from perfectly dry; not "wet", of course, but the air humidity must be mostly 80% relative...your desert environment must be better than that!
Under those conditions, I've had some batches stored for quite over one year.
Not that this is a goal onto itself: green coffee does NOT improve with age.
Good luck!
Paul