how often do you guys change the inner filter on your donaldson type air filters? i blow the outer one out with compressed air every 50hrs and when it gets to looking bad i replace it. the inner one however stays pretty much clean. how long do you let it go before you change it? i've had the original one on there for 2 years so far and it still looks clean. i'm just looking for a ballpark figure or for ideas about how you guys do things. thanks.