A lot depends on how you define "clean." Using one mug, I will usually do an entree, then dessert or whatever, followed up with a nice cup of tea brewed in the same mug. When that is drained, I declare it "clean" and move on. If I am really fastidious, I will follow the tea with a sip of plain water. This is OK as long as just one person is using the mug, especially since my "cooking" consists of boiling water to prepare the food.

Perhaps he is eating MREs. Not too many dishes to do with them.