Depending on the tobacco, as little as one. A strong Lakeland, like Ennerdale for example, might leave a ghost in your pipe after one bowl. This doesn't mean it's ghosted forever, it just means that you will be tasting Ennerdales' essence in the next several bowls of whatever else you are smoking. However, a pipe can be permanently ghosted after (X) number of bowls, it all depends on the strength of the tobacco that has been smoked in the pipe and the material the pipe is constructed of, be it Briar, Meerschaum, Morta, Cob, etc..