I too have wondered about when to change, what type to buy and other questions about an A/C filter.
After much research I found that A/C filters are measured by "MERV's". You can spend considerably more by buying a nice filter than a cheap one as having problems sometimes with asthma the better filter is better for your lungs and dust problems.
Also by not changing a high quality filter that collects many more foreign particles can cause the air not to circulate and make your A/C work twice as hard but will trap pollen, mold and not be like regular filters that let 90% of EVERYTHING pass thru it.
Bottom line..... How much are your lungs worth?
Found that buying "online" can save you money and in my opinion FILTRETE has the best filter. Hope this helps...
http://www.bestacfilter.com/faq.html
http://www.google.com/products?hl=en...num=4&ct=title