I'm not even going to tell you how many KM we have on our E90 335i (it's ridiculously low). However, we still have the original battery on our '07, and I use the battery tender whenever the car will be sitting for more than just a few days, even in the summer driving months.
2011 E90 M3 Alpine White on Fox Red
2007 E90 335I Graphite on Terra
2005 E83 3.0I Mocha on Black (Gone and forgotten)