My and my family's experience over a range of different makes of car is that the original factory fitted battery lasts for ten to twelve years and replacement batteries die soon after the warranty expires.

The exception was my wife's GETZ where the battery died after four years from first registration.

Edited 25 September, 201510 yr by pikpilot

Do you have a cd changer or any other entertainment equipment ? I ask as I had an A4 where the battery kept going flat , checked all the usual suspects and one day when it was really quiet after locking the doors I heard a noise from the boot , it was the 6 CD changer constantly cycling. Put a meter on it and it had a massive current draw. Changed the fuse to ignition live and voila !!