The traditional metric was enough to last a week. That's the origin of the seven day set. It really depends on your smoking habits. Some smokers only use a pipe once a day, so if they smoke three or four bowls a day they use three or four pipes. Others, me included, smoke several bowls in the same pipe during the course of a day. Some smokers only smoke one blend in a bowl so as to season it with that blend's flavor and thus avoid any cross pollination of flavors. I have pipes that I use for Virginias, Va/Pers, and their variants, and others for Latakia and Orientals. I also have pipes of different chamber sizes as some blends do better in a larger bowl and others do better in a smaller bowl. Whatever you decide to do, make sure that you thoroughly clean and dry your pipe at the end of a days use.