As weasel said, it depends on many parameters. I have it on only after driving uphills in a sporty way for 20 minutes... it's always the same running engine temperature on the gauge, but for some reason, after putting stress on the engine, the aux remains on after i turn off the X.

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BMW X5 3.0si 2008
Premium Package
Light Package
Navigation
Reverse Camera
Rear Sensor (retro-fit)
Comfort Access
Spare Wheel
Power folding mirrors
Voice Control w Bluetooth
Power trunklid

BMW X3 3.0i 2006
M Aerokit
M Sport Package
Navigation System
BT Handsfree
Xenon HID

BMW X5 3.0i 2001 --- SOLD