Dirty Aprilaire filter, should I change it?

Consumer question:

I had installed a Heat and Air unit in Oct. 2009. At the same time I also installed the Aprilaire filter system. I was told that the filter would need replacing after about a year. They replaced it on one of my seasonal services in Nov. of 2010. My brochure that I received with my work says that the filter will last 1-2 years, however I just had my seasonal check and the service man replaced the filter again. I told him they replaced it in Nov. of 2010, and his reply was " I don't know, it was pretty dirty."

Aprilaires' response:

Depending on your application and the tonnage of your system, you may want to discuss with your installing contractor. We typically recommend the following:

MODEL SERIES	INTERMITTENT FAN	CONTINUOUS FAN
2120/2140	Once every 6 mo – Once per year	Once per year
2200/2400	Once every 1-2 years	Once per year
2210, 2300, 2410	Once every 1-2 years	Once per year
4200, 4300, 4400	Once per year	Once per year
5000	Once per year	Once per year