You'd never guess how quickly an air filter get dirty to the point where they start costing you power, and often it's the last place people would look into when maintaining the cars. The most important thing when maintaining the car is to check every single section which is really important although sometimes people would think it is not important.
As a general rule of thumb, the move 'hi-flow' into your inlet, the faster it will get dirty; and the more often it will need cleaning. Every 5000 km as a minimum is worth the effort. If you don't have at least an aftermarket panel filter then invest Rm100 in one. Just a suggestion from me.