Being a Mechanic for 10years I can say AVERAGE is about 4 years. Mind you some can't make it a year and I saw one that was about 10 years old. So average does not mean much if your 2 year old battery will not start your car.
Also there are not that many people that make Batterys. So saying brand A is better then B might be dumb as they could be from teh SAME plant.
Places I like are Autozone, Sams, and Advance. Those seem to have a easy return setup. I would not recommand a Sears battery. Most of them are very hard to get your warrenty through, and most of the time they want to charge you to test your charging system before they will even look at the battery, etc...
The main causes fro a battery to not last long is it not being tied down correct. When a battery moves around it can cause the plates to come free. Also keep a eye on the fluid level if not maintaince free. And lastly keep it charged and don't drain it, so don't leave a light on or run soemthing with the car not running.