It is recommended that you run the battery down at least once a month...I've been using it plugged into A/C the majority of the time. The tech at the Microsoft Store said it doesn't hurt it to be plugged in all the time, and that the battery has a limited number of re-charges. I haven't had the opportunity to drain the battery and get a time yet.
Battery care
- Operating temperature. Surface is designed to work between 32°F and 95°F (or 0°C to 35°C). Lithium-ion batteries are sensitive to high temperatures, so keep your Surface out of the sun and don’t leave it in a hot car.
- Recharge anytime. The battery doesn’t need to be empty or low before you recharge. You can recharge the battery whenever you’d like. However, it’s best to let the battery run to below 10 percent at least once per month before you recharge it.
http://www.microsoft.com/surface/en-us/support/hardware-and-drivers/battery-and-power-pro