I haven’t gotten any stains on my mugs yet, but if I do I guess it will depend on how bad the staining is. If it’s light I’ll keep using it, if it gets dark and really bad I’d probably replace it.
I'd drink from a stained mug, no biggie. But if i ever had a problem with it, I'd just remove the stains. My husband does brewing as a hobby, and he has some pretty good cleaners for the brew equipment that also work to remove stains. I had a mug once that was pretty stained from tea. He got it like new, no more stains.
I don't drink coffee so I haven't had any trouble with this before. Occasionally tea might leaves stains, so I wash them a little more thoroughly until they come out.
My mugs don't have any stains. Just give the mugs a quick wash and/or fill the mugs with water if you don't feel like washing it properly right after use and you'll never have a problem with stains!
i've managed to avoid getting any stains on my mugs so far, but i do think if they got bad i'd stop using the mug unless i was really really attached to it. fingers crossed i can avoid that scenario entirely though - i've never encountered it as an issue