This is !tea@feddit.uk after all, so it fits, especially as !mugs@feddit.uk got deleted so we are picking up the tea drinking receptacle slack for now.
Oh I don't clean it. Maybe once a month but if I want to get it spotless I need to soak it in bleach overnight. So it's heavily crusted in coffee stains and dried coffee milk bits round the inside edge.
Yeah I'm aware it's gross but it's not killed me. It had stopped me from drinking tea though until I got this mug which is only allowed tea in it.
I had a colleague like this. He had a camping mug that he wouldn't clean - literally had an eighth of an inch of stains on the inside of it. A stainless steel cup with what looked like a void inside.
He loved it, too. So much so, that if you dared to clean it, the dude would go batshit crazy.