Before and after Lans, as well as any major hardware upgrades. On average if none of the previous take place I would say 2-3 months. Working at a computer sales/repair store I've seen some incredibly dirty systems, the ones that smell like ash trays are by far the worse. I find it hard not to laugh when someone asks me if that could be why their PSU stopped working, well the fan is seized and the rear exhaust is completely clogged with brown tar... I wounder. Makes me feel better about not smoking and fear for those who do.