Unless you drive on a lot of dirt roads, you should be able to go 40-50k before you need to change it. I changed my first one at 12k per the service interval and it was still very clean. I changed the next one after it had 24k on it and it was still very clean. I am going to change the next one when it has 50k on it.
I have changed the air filter twice, first filter had 36000 the next had 41000 and I put quite a few miles on dirt roads. Don't know why but the filters both look as clean or cleaner than Dale C's with only 24000. My current plan is to have the next filter change at the dealler this fall along with the ECM.
Mine was just a bit cleaner than Dale's. This was at 17,000 miles. I can't see leaving one in for 40,000 or 50,000 miles. Seeing what mine looked like at 17,000, I'll go no longer than 25,000 between changes.