I used half a lemon after I had already used it in the morning for my hot lemon drink as there's no point wasting the juice as well so it can serve two purposes. I put my half used lemon in a breakfast bowl of water and put it in the microwave for 2 minutes and then I wiped the inside of the microwave with a cloth and as it was pretty dirty I gave it another 2 minutes in the microwave. The steam from the hot water and lemon breaks down the grime and you can literally wipe away your hardened splatters as easy as that.
I found out this simple method about 4 years ago and each time I clean my microwave with this method I can't believe how easy it is. Afterwards I just throw away the used lemon and it's nice and clean to start cooking again. It always makes me feel really good when I do this and seriously you can clean it in 5 minutes however if you wanted to take your time 10 mins max and it's nice and clean. It beats scrubbing hard dried food off the insides of the microwave. No chemicals and not too much elbow grease and you have a nice sparkling microwave oven ready to use. Everyone can find a spare 5-10 minutes to clean the microwave.