When it’s no longer pleasurable, I’m done. Life’s too short and pipe tobacco too affordable here in the USA to do otherwise.
Don’t fall victim to the oft-mentioned idea that you need to smoke to the bottom of the bowl for the benefit of the pipe. Hogwash, IMHO.
Can't give a definitive answer. Bedtime is one indicator. Taste another. You just know when it' time to swap a pipe out for another or ... go to bed. If the last pipe isn't finished, it'll be there to accompany that first cup in the morning. Or, maybe after the first cigarette.