How often to change the brake pads?

I drive a Nissan sentra 04,I am hearing a constant scratching sound from the front wheels,The maintanance manual says it is the audible reminder to change the brake pads,How long(avg in Kms) a brake pad lasts in city driving??How often to change the brake pads?it all depends on the material making up the pad, if its cheap semi metallic combined with persistent sudden braking, you can run down pads in less than 6 months, invest on carbon ceramic pads and don't do sudden braking.