I want to say I had my 12 V battery replaced maybe around 75,000 miles? I seem to recall that because the cost is fairly significant for a new AGM battery and it should get paired. Or I'm misremembering because I know for sure I've had to replace the 12 V battery once or twice on previous ICE MINIs I've owned. If I did have it done it would have been at the dealer.
My SE lives in an unheated garage, but sits exposed all day long when I'm at work. And our winters do get extremely cold, so 12 V batteries usually only last 2-3 years. But the SE doesn't need cold cranking amps to start, so I don't think the stress is as great on the batteries.