X5 Carbon air filter
 

Does anyone know if the carbon air filter in our X5 are covered under the 4 year maintenance warranty?

cabin air filter - Yes but they don't replace it with the Activated Carbon microfilters, just regular ones... But if you have a good relationship with Dealer then you can ask them to use the Activated Carbon replacements...

Do you know roughly how long the Activated Carbon microfilters lasts. Is there a sensor that lets us know when it needs replacement?

No sensor... They normally change it during inspection I and II which is 15k miles... I normally change mine every 18mos. by buying the part myself.