I try to do twice/year, if I go a year they tend to look kinda plugged up. Maybe they aren't, but they'll have leaves, acorns and look awful. But a year is 25kmiles and the cars were parked outside--now that they are parked indoors maybe once/year will be just fine.
I try to buy in bulk so as to knock down the price. I don't feel a need for fancy but I wouldn't rule them out either--when my allergies are kicking I really do like the notion of having clean filtered air in the car (a good pollen filter that is).