AC on Battery

So I'm at the dealer today in the hope they will update the backfire issue. The SA said it takes a long time (to hook the thing to a computer and walk away???). Anyway, I drained the battery so if they had to go for a joy ride they could experience the issue quickly. I got to wondering with my drained battery about how long the AC would run on almost no battery while sitting at a traffic light. I know a lot of start/stop cars will start the engine after a minute or so sitting with the AC on. So how long will the Volt be able to run the AC at a light if there is effectively no battery left before it starts the engine?