I just replaced the one in the 2001 I bought with 25 K miles on it. Obviously it had not been done since new. The old one was disgusting- nearly blocked shut by ugly crap. Ventilation, heater, and A/C all work MUCH better with new one. I'm planning on a 10K mile replacement interval- the schedule in the owner's manual is way too infrequent. Especially true if you do a lot of city driving and/or keep the fan running most of the time. Happy Trails-

 

Good idea, my air is starting to smell funky

 

This can also be caused by organisms growing on the moist A/C coils. It is a good habit to turn off the A/C a few miles from your destination(leaving the fan on high) to dry things out before parking.