Depending on the state of charge (which can vary dramatically), options on the vehicle, and ambient temperature a X3 battery can be depleted in
as little as 18 days.
Comparisons with other vehicle brands & models are totally irrelevant. The X3 BER system changes everything. This has been discussed and explained in this thread:
X3 LOW BATTERY
Posts No. 20 & 22 address this issue in detail.