Could be too long a time, but going brittle doesn't usually happen with an OEM filter. I would investigate whether it is a jobber filter (not OEM), whether it is labelled BMW or not as it could be a counterfeit part. Also, I wonder whether there was too much heat, but I would expect other problems as well in that case (if you had overheating, and had to fix something else already, then that may explain the brittle filter).

OEM filters don't have any problem at all if you follow the change intervals recommended, which are the first of 15,000 miles and up to 2 years in service (depending on year of manufacture).

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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