There is no good statistical data to be able to answer your question. Failures don't fit a traditional bell curve. Some owners have run close to 300,000 miles. Others have barely made it through the warranty period. It won't likely wear out, it will just fail.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue