Extremes in temperature at low charge are going to affect it more than at more moderate temperatures. For maximum life, follow the operator's manual instructions, and either leave it on charge, or at least charge it up before letting it sit. If it does get too low, and this is more true for the 12vdc battery, nothing will work until that comes up - the lead-acid, even the AGM type doesn't like low charge, and it must be functional to bring the car up. If the car is plugged in, it will keep the 12vdc battery charged as well as the main one. It will not try to charge the 12vdc battery other than when plugged in or in ready state, as far as I can tell, though.
The 12vdc battery is what maintains the run-time stuff like the alarm, comfort access, clocks, and various other bits like the cellular connection. So, like most modern cars, there is some constant but small drain on that battery. The main battery when off doesn't really have much of any load on it, and doesn't self-discharge anywhere near as fast as the AGM one, but still will. Once the main battery reaches some critical point, it won't be able to be salvaged, but I don't know where that is, and as I said, it will vary by the ambient temperature some.
IOW, you'll have less likely problems in a moderate climate than say the desert or arctic, but it will still happen. Now, if you lived in Death Valley, or Anchorage, or the northern half of the US in the winter...expect problems sooner.
2011 535i x-drive GT, 2014 i3 BEV