Battery life depends on many factors, but usually 3-4 years. 1 price paid for the battery, the higher the price the longer it should last. there are 36, 60, 72 grades on batteries, the higher the number the better it is. 2. conditions, extreme cold or hot will lessen a battery's life 3. usage, short drives where it can't be recharged completely, accessories running when engine is off will do this also. Lead/acid batteries have only a certain number of times they can be discharges and recharged before they wear out, if you want your battery to last longer you should pay a little more for it, and use the warranty if it does not live up to it's life expectancy.

You can get a cheap battery at Wal-Mart but I would suggest going to Advance Auto Parts or Auto Zone to replace a battery. They will even put the battery in your car for free.