I honestly do not know how many miles are on my air filter.... I have put close to 30k on my truck and have not changed it. I am not sure if the PO ever changed it, but I am sure they did.... the maintenance records I have on it there were pretty thorough about the maintenance. I have mileage ### next to all the "milestones" in the owners manual for all the services right up until before I bought it (I am the second owner).
I did some research on the air filter. The stock filter (the one with the blue filter material) is made by Baldwin for Ford. If I remember the Ford specs correctly, that filter is capable of holding something like 3 POUNDS of contaminates before it become clogged by the Ford'sSpecs. So I would say that unless you are driving around in a sand or dust storm constantly, they will last quite a long time.
I double checked the 6.0 Bible and it says that "The air filter is capable of holding 1600 grams of particulates before needing replacement" So doing a little math... 1600 Grams = 3.53 Pounds. That is a lot of dust and dirt........