Here is Teslas response:
The earliest Roadsters will take over two months to discharge if parked at a 50 percent charge without being plugged in. From that starting point, Tesla has consistently innovated and improved our battery technology. For example, a Model S battery parked with 50 percent charge would approach full discharge only after about 12 months. Model S batteries also have the ability to protect themselves as they approach very low charge levels by going into a “deep sleep” mode that lowers the loss even further. A Model S will not allow its battery to fall below about 5 percent charge. At that point the car can still sit for many months. Of course you can drive a Model S to 0 percent charge, but even in that circumstance, if you plug it in within 30 days, the battery will recover normally.