My personal choice is to allow at least a week before I return to the same pipe. I might smoke 3-5 bowls in it one day, but then it goes in the rack for at least a week. But it's often there much longer, since I dedicate pipes to specific types of blends. Seven is my minimum number for a specific category of blend. That means I have at least 7 pipes for English, for Balkans (which is one of my favorite types of blends), for Penzance (my go-to favorite tobacco), for straight Virginias, and for Va/burley blends. The only variation is that I have only 4 or 5 pipes dedicated to aromatics, because I hardly ever smoke them. (I have somewhere around 40 altogether.)
One disadvantage of my system is that I think it takes about 3 bowlfuls of a new/different blend in a pipe before the previous blend is gone and you can really start to taste the new one. It helps when you dedicate pipes to certain categories, but that doesn't entirely solve the problem. I mean, I may go from Caravan in a Balkan pipe to then smoking Presbyterian in my Balkan pipes. Presbyterian isn't a Balkan at all, but I choose those pipes because it focuses so strongly on oriental flavors, which Balkans do too. But when I move from Presbyterian to something else in those pipes, I need to have at least three bowlfuls of that "something else" in each one before the Presbyterian ghosts are going to dissipate. And that doesn't always happen, since I'm rotating pipes, maybe moving to different blends at the same time.
Yes, it does get confusing!
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Bob