How much longer can the battery last?

Since I bought my car in Oct 2010 I have used it as a second car and I have not driven it much. I will drive it every 5 days or so for anywhere between 18-30 miles and then it will sit for another 5 days. So far the car has started up well each time and going on a year and a half of using the battery like this I have not had any failures and it tests out well. How much longer can a battery last like this? Is that something that can be known? I have heard that a battery used like this will last but not as long as if I was to drive it everyday. Is there anyway to predict how much longer the battery will last or is it random?