You can change it whenever you feel but I think that the price might have
you rethink that.

If you look closely, you will see that the filter is equipped with a
restriction gauge... he more yellow band you can see, the higher the
restriction...

Our region can be very dusty in the "summer" and we haven't seen the need to
replace any of these filters yet.

Well I tried replying earlier, but it has not appeared, so here goes
again:

Take a look at
http://www.visteon.com/utils/whitepapers/2005_01_1139.pdf - this is an
SAE paper describing the Focus Zero Maintenance Air Induction System.

Be glad you have a filter that lets you know when it is restricted. I
came across the following on the internet (it was primarily in regards
to heavy truck air filters, but the principles are the same for cars):

"The SAE paper by Marty Barris (SAE Technical Paper No. 952557 titled:
'Total FiltrationTM: The Influence of Filter Selection on Engine Wear,
Emissions and Performance' ) offers two important conclusions:

"The level of ambient dust concentration can easily overwhelm other
factors in terms of influence on engine wear. It is therefore
important to match the air intake filtration system design to the
anticipated ambient exposure. And here's an interesting one-:

"Too frequent air filter change intervals can double engine wear rate,
especially if changed within the first 30% of the air filter's life.

"The practice of removing an air filter element to blow it clean is
plain stupid. This malpractice, which can lead to engine wear rates
being trebled, still occurs on the false economy basis of trying to
extend the service life of the air filter element. A well meaning but
badly trained technician thinks he is being thorough in servicing the
air filter, or replacing it prior to reaching the optimum life of the
element. This happens in workshops without supervisors even being
aware of it."

Ed