when should filter be changed

read your owner's manual. follow recommendations, if you want to extend the life of your vehicle.

After a while, your air filter will get full of dust, especially when you drive on country roads, or spring time, when you live in areas where they dump loads of sand on the roads during the winter. see it like this: the filter has millions of little holes, just large enough to allow a certain amount of air through, yet block out particles, like sand and dust. but the sand and dust stick just over these holes, from the air suction, and get lodged in there, without falling off. so, let's say you have 500 000 000 holes (totaly random number), well, after 2 years of normal driving, you only have 2 000 000 little holes left, to allow air though. now the motor needs a certain percentage of air mixed in with the fuel in the combustion chamber, to have the best output of energy to make you car advance. but air can be compressed, and it also can be stretched. so by the time the cylinder has finished going down, and finished sucking air, it doesn't have the right amount to have a proper explosion. so you lose horse power, have a bad fuel economy (cause the computer tries to compensate with more fuel), and add more wear on the system. also, yiou can develop problems related to vacuum pressure, all depending what is controled by it on you car. SO: replace your air filter as recommended by you automobile fabricator, and if you don't know what the recommended period is, do it every 48 months, minimally

Do you mean 4 months, or 48 months?

sorry, 24 months. i'm still sleepy. every two years, in other words

Thanks