Here in Phoenix I'm pretty regular, I keep a note on my whiteboard as to the date they were last changed in my vehicles. It's a big dust bowl after all
You can also get a K&N cabin air filter, one that is washable/reusable. I put this in my Prius, without knowing the years of your vehicles you'll have to narrow it down yourself:
https://www.amazon.com/VF2001-Washable- ... 00NEL6HK4/They also sell a recharge kit you can use once it gets dirty. When it's brand new you just install it, no need to pre-treat it or anything.
https://www.amazon.com/99-6000-Cabin-Fi ... ref=sr_1_2I'd judge your cabin air filter change by your environment (desert/lake front/high mountains/etc) and by looks. Just pull it out and see if it needs it