but its dependent on how much the car is driven and the environment its driven in..
Heres the link to amsoil air filter if you want to go that route for your cobb intake.
http://www.mazdas247.com/forum/showthread.php?t=123726397
also heres apic of it on my car.
the cobb filter is a good filter. but the dryflows are wash with warm water mild soap and call it good.