I have had my car for about a year and a half now and I'm at almost 37k miles. I haven't looked at the air filter change interval on the car and am not really all that concerned. I've only had the dealer doing oil changes and (stupidly) was assuming the air filter would get done along with the oil change. But I also figured they'd bring it up once it hit the interval anyway.
Anyway, not really the point of this post. I just pulled the air filter cause I've felt like I'm not getting what I used to out of the car. And boy am I glad I pulled it! I put it up to a 75w outdoor flood and I can't hardly see anything through it. With as dirty as this thing was, I am SURE to get better performance and mileage back into this car!

 

If it is that clogged then you might well get somewhat better performance out of your engine, but your mileage won't change one bit.