I use my plastic and metal coffee cup about 6 or 7 times (i.e. pretty much every day of the week) between washings.  My rationale is that I'm putting the exact same thing in it day after day, so the very thing that qualifies as making it dirty is just an old version of what I'll be putting in it again.  The only reason I end up washing it is because I'm afraid of bacteria and mold-grown super monsters that will likely eat my brains when I'm sleeping.  Unlike some people in my household, I don't feel the need to wash it after a single use. #products