I think this depends on the blends you smoke and the specific pipe we're talking about. I have briars that can easily 3-4 bowls a day before needing a couple of days' rest, while some really only seem to like a single bowl. Overall, though, I don't think briar is at all fragile or finicky. It can take a lot of abuse before it complains. I grew up around guys who smoked all day every day had only 2 -3 pipes in their "rotation." Mind you, they were smoking mostly old-school burley blends, and those blends seem easier on a briar than the boutique blends that many of us favor. Just my two cents, of course -- I really don't think there is a straightforward answer to this. Just experiment and do what works for you!