I change mine every six months.
The Air 11 (and Air 10) User Guide states:
Check the air filter and replace it every six months. Replace it more often if there are any holes or blockages by dirt or dust
If you're in a bind and without spares, take it out, note the orientation of it (dusty side out), flick it a few times with your finger and put it back in, ensuring the same side remains on the out side.
The last thing you want to do is blow all the debris that has been collecting on the one side into the machine.
I use authentic ResMed, but not because I am brand loyal, I'm not. I just bought a boat load of them. Many use off-brand and assure they work fine. I accept that.
A used filter actually performs (filters) better than a brand new filter. Changing them too often gets you nothing.