How long does - on average - does a car battery last?

OK this is based on you maintaining the battery and the charging system properly. On average 5 to 7 years for the average domestic vehicle. Now when I say maintaining your battery and charging system properly, this means yearly maintenance on the battery cells, battery posts and cables and checking the alternator belt tension. See everyone is told to get maintenance free batteries however in actual fact they still need to be maintained. As the charging system recharges the battery, the acid in the battery (sulfuric acid) starts to boil as it takes on a charge. When this happens the fluid levels in the cells will, over time start to drop exposing the tops of the cells to the air and this exposure will start to corrode the tops plates of the cells. Once the cells start to corrode they will start to loose the ability to hold a charge due to internal short circuiting inside the battery. If you are mechanically inclined you can check these fluid levels and top the cells up with distilled water to the bottom of the plastic ring inside the cell opening. If you are not so inclined don't attempt to do battery maintenance yourself, sulfuric acid is not friendly to clothing, skin, eyeballs or any other portion of the human anatomy. Get you charging system and battery checked by your local mechanic or service station. This will save you grief down the road and extend the life of your battery big time