I'ver never heard of a case where a PC broke down because of dirt and I've been building PC since 1990. I clean my PC maybe once every 2 years. Depende talaga kung gaanong kadumi yung kwarto mo. Ang dapat mo lang tingnan ay yung fan ng powersupply, cpu fan, atsaka case fan. Make sure na hindi naka-obstruct yung flow ng air. Dapat walang nakasabit na alikabok. Other than that, the less you touch (sa loob ng PC), the better. Mga wires mo sa loob kailangan nakatali lahat para good air-flow.

I clean my monitor everytime may fingerprint or food stains that starts to stare back at me.

I don't think keyboard needs any cleaning. I never clean my keyboard. Sobrang nasty na talaga itong cheapo keyboard ko pero no problem pa rin. Meron akong keyboard tumagal ng 5 years without cleaning, pinalitan ko lang ng mas bagong modelo.