a good indication of engine needs decarbonizing is when you feel sluggish or lack of power. usually mileage gives a good indication, once every 30k km.

a good way to reduce carbon build up, is not to turn off the engine right after a drive, let it cool down in idle abit, then turn off, this will lower the temperature difference between the intake valve & combustion camber, which will lower the chance of carbon build ups.