Being the idiot I am, when servicing my car over the last three years, I left the pollen filter off the list. Anyway, I changed it on the weekend, and this is what came out. Safe to say I'll be changing it every year from now on!
Unless you live in an excessively dusty area, I'd change it every 2 years, that the recommended interval iirc. but you will notice your windscreen defogging a little quicker I bet!