Run on battery whenever you need to and plug it in whenever you can. You can plug or unplug any time you need to, regardless of the charged percentage, and you never need to completely drain your battery. Just make sure you don't run on AC power exclusively, as your battery needs to be used regularly to stay healthy.
There are many factors that impact your battery life. See the BATTERY LIFE FROM A CHARGE section of the following link for details, including tips on how to maximize your battery life.
The link below should answer most, if not all, of your battery/charging questions. If you haven't already done so, I highly recommend you take the time to read it.