Never run a LithiumIon battery dead. Kills it early.
I keep mine plugged in when near an outlet, but run it on battery down to 40% at least once a month. Still have 101% of capacity at 18 months anf 35 cycles.
Usage is advertised at 7 hours, but that is limited anything (no Flash, simple web browser, no complex graphics, etc). Reality is about 5 hours, less under heavy usage.
Current Macs don't need to be calibrated though it is a good idea to use them on battery at least once every two months. On the last Saturday of the month I use the computer on battery just to make sure it gets that much use since I tend to use it plugged in most of the time.
How long the battery lasts depends entirely on how you use it. I can totally use the battery in about 2-2.5 hours in heavy use - playing a game or watching streamed video. Doing word processing with WiFi and BlueTooth off I can go a bit more than 6 hours.