Its recommendable to physically inspect your air filter every 5000km. If you feel the air filter is filled with dust, then I'll recommend you change it. I have city 2009 and the milage on the odo is 21,000 and its my 3rd air filter. The best way is to inspect and change accordingly. Afterall, pakistani roads n conditions are always dusty.
By the way, the new technology of the airfilter is that its a bit wet. so it doesnt allow dust suck in. the price of the air filter is 1,260 rupees. Remember, clean air filter = better milage
Best of luck.