I've checked the battery drain on my 04 Coupe which should be representative of what you should be getting. I do have an aftermarket head unit/ipod setup but that should have drain to the factory system.
The current drain is around 114mA with the car locked i.e. alarm armed. There are odd peaks to 156mA, presumable the ultrasonic scanning periodically. So assuming you have a fully charged stock battery which has a capacity of 74Ah and taking the worst case 156mA drain you should get:

74/0.156 = 474 hours or 474/24 = 19.75 days until the battery is flat.

If you take the 114nA figure you get around 27 days.

But in both cases bear in mind that the will not be enough charge to actually start the car well before the total discharge point. Also the battery is unlikely to be charged to its specified capacity, especially in the winter months.