ed850,
Typically batteries don't last more then four to five years on todays modern cars. The demand placed on these batteries with the many power sources has shortened the life compared to years ago when you could get eight or more years out of a battery. The electrical demands have become so great, that the auto industry will be adopting a 42 volt system soon which will see things like compressors and pumps electrically driven, doing away with belts. Having said that, you can wait untill you see early signs of wear (engine turning over slower at start, headlamps dimming when additional electrical sources are turned on) or when the battery warning lamp starts to appear. No, you don't have to use an OEM battery, just make sure the replacement battery fits the compartment and has the minimum rated cranking amps needed for the car. Purchasing an OEM battery takes any guesswork out of the purchase. Having said that, my OEM battery is going on five years old and I intend to replace it this fall before going into winter...