Whew, that's a good one. My answer would be "as many as it takes". I've got a pipe that really stunk at first and didn't seem to smoke right until (I'm guessing) at least fifty bowls, but that was a standout exception. For me, pipes seem to smoke better and better right up until the time when I realize I waited too long for a deep clean. Most of them seem pretty well broken in after twenty or so bowls, and 'close enough' after ten or so. Don't ask about the ones that are dip stained; I don't want to talk about it. I'm looking forward to seeing other opinions on this one!