2007 Ford Focus engine air filter

These air filters do not require any maintenance, just replacement.
There might be an aftermarket replacement air filter housing that allows you to have a serviceable air filter.
Dorman is the manufacture
The part number is: 258-519
These air filters function just like any other air filter just a difference in design. But as you know they did away with this design. I don't think is was a good decision by the manufacture.
I wouldn't initially suspect your air filter as the cause of your decrease fuel economy. Unless it was completely plugged. Removing the filter housing and checking for lots of debri blowing compressed air through it. It would have to be extremely plugged up to cause an issue.
I would look for low tire pressure, dragging brakes, reduced sensor sensitivity, check engine light and poor fuel quality.
Some other things to consider that will use more fuel.
Cold ambient temperatures make the engine use more fuel to maintain the operating temperature. Using the air conditioner in hot climates uses more fuel due to added an extra load on the engine. If you have something adding more drag and less aerodynamic. Like a cargo rack or missing bumper pieces.
Hope this answers your questions.

I don't believe in leaving any filter in a vehicle for a long time. replace it with a disposable one as the post before recomended