I have an open element type paper filter, ya know the one that looks like a big coffee can, how often does everyone change them? Do you just use your best descretion?
I also do mine about once a year. I drove an average of 20-25k miles a year if that helps at all too. I just actually changed it for the first time (thought it was REALLY clogged) and honestly felt no difference (maybe she still have some life in er), but rather be safe than sorry with a dusted motor.
Your type of driving also affects when it should be changed...meaning do you drive mostly on the road? Or are in dusty conditions a lot?
I will change mine someday when it looks like it needs it. Running about 2500 miles a year I am thinking it should last a while. It has been on there for about 3 years and still looks good
Short answer: Use your own discretion. If it looks like it needs changed then change it..