Do you bring a coffee thermos or to-go mug to work every day? When's the last time you washed that thing? Not just a rinse, but an actual soak and scrub with soap?
According to a Mashable article, once a week is acceptable.
Here's what the KeepCup people say about it:
If you're using your cup everyday, then "a quick rinse will do. ... We recommend a thorough clean once a week," reads the care instructions on the website.
Click here to read more.