I’ve been bad about keeping an eye on the Cabin Air Filter / A/C filter in my 2008 IS 350. I finally got around to changing it on 6/10/2010, around a year and a half after ownership (bought the car new Jan 2009) and my car was at 25961 miles:

At the time that I did it, I noticed my A/C was barely working. I had to keep the A/C really high and even then, it was still a little warm on the really hot days.
I had bought the BLITZ PERFORMANCE
HYBRID AC FILTER 18737 LEXUS IS350 off of Ebay for $46.75 + $16.38 shipping a weekish prior.
The big thing to remember is that the A/C MUST be set to Auto:

That’s the one thing that’s been beaten to me on my.is and clublexus.com.
I actually struggled a bit with the installation steps. It was supposed to be easy to remove the things in the glove compartment to have access to the back, but took me a while to fight w/ rack and get it out.
Once I finally got to the back and opened everything up, I noticed a lot of the smaller papers I had in my car had slipped into the back (for example, envelopes, chevy chase bank deposit slips, etc). I need to remember to check the Cabin Air Filter area often.
I have to double check how often I am supposed to look into the Blitz Cabin Air Filter. I believe the proper procedure after I clean (not sure how to go about that) is to “expose the filter to a good 6 hrs of sun to give the Titanium Oxide time to do it’s work.“
This is what my Cabin Air Filter looked like after I Took it out:



I think I can still use it after cleaning it according to this post on my.is, but I need to find more concrete info on this.