It seems to me that as a general rule you get the remainder of the month of purchase plus 13 full months. However, some people have reported that if they purchase it during the first week of a month they will get the remainder of the month plus 12 full months.
So if you make the purchase in late November of 2011 it will run through the end of January 2013, but if you buy it at the very beginning of December it may also run only through the end of January 2013 but may run through the end of February.