I don't know the average life of a filter, but they last a long time if you clean them every day.
I run the K&N style filters (metal screen). I found I have the best success cleaning them in a sink full of strong dish water. I shake them pretty hard in all directions while they are under the water, then rinse them off and shake them out, set them on the mantle and they are ready to go when I am.
There is probably a better way, but this works real well. I've worked on forest fires where the sut realy stops them up, and the dishwater cleans them better than anything I've found.
Don't use a brush on them, it will realy eat them up fast. If trash is stuck in the bottom of the pleats, just keep shaking it in the dishwater it will come out.