I've never gotten less than 10 years out of the OEM battery in previous cars. If the battery fails in the BMW, I'm putting a $100 unit in and not registering anything. BTW, it is very simple to check the out the battery or the alternator with no more than a multi-meter:
The link above is for the alternator which should always be checked. A link is given to check out the battery.
You'll notice these are very simple tests anyone can do. Unfortunately, when you go to a local shop to have your battery checked out, the shop will do a static test with some electronic gadget which will always indicate you need a new battery. This has been a running con for years, but I have to admit BMW has brought it to a high art form!