Depends on the humidity.

If it is cold, and wet, I leave the air conditioner one all year. Windows clear faster as the A/C functions as a dehumidifier.

If it is dry (ie in the far north) and cold, I turn the A/C off, but make sure to exercise it for 10 minutes every month to prevent the compressor seals drying out. That can cause a failure of the compressor ($$$).

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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