Manual clearly states that driving conditions determine how long the DEF fluid will last. For some that means 10k miles. For others that means 5k miles.

You will eventually get a 1,000 mile warning/message, in which the vehicle informs you that you have 1,000 miles to fill the DEF fluid, at which point you should take the vehicle to your dealer for service.

You can fill the DEF on your own, in a pinch, but it appears that many dealers are flushing the DEF reservoirs (there are 2), before refilling an X5 35d with clean DEF fluid.