Once per 6 months for me if I decide to reserve 1 whole day during the weekend.
I will disassemble my entire rig and clean each component one by one. And slowly reassemble everything back.
It takes a long time cos I don't have air blower or compressed air. I wipe by using paper towels, usually the fan blades, HSF and the interior of the casing. Areas which is safer from electrostatic discharge, I will use vacuum cleaner.
It feels good to have a clean PC minimal of dust. But sometimes u also wonder if it's worth it to do it too often. Quite back breaking task sometimes especially if you have a cramp room to clean.