HELP with micro filter

[Harman1]

Does any one know were the cabin micro filter goes and how to change it.
Also how often has everyone been chaing the air filter.

 

[330i and like it]

It is located under the hood directly under the windshield. You will see a long thin black cover with 3 knobs. Undo and the cover will lift up.

I have changed might just once - I don't really see the point.