I'm quite ashamed to admit this but mines around 3 tines a year!
I throw out old food etc on a weekly basis, but a thorough clean is once in a blue moon.
Microwave is the same, every few months when it's full of splashes and stray bits of food I microwave a bowl of water. Let it steam the inside for a bit then use antibacterial to clean it out with.
Am I disgusting?! I have 2 young children and jobs like these, washing down skirting boards etc I struggle to find time for.
No excuse really as I did the microwave as dinner was cooking, maybe I can make it a habit to utilise the time I have whilst things are cooking away...