Until it's not tired anymore
Sorry, old joke. The purpose of "resting" a pipe between smokes is to give it time to dry out. As you smoke condensation, the moisture in your breath and moisture in the tobacco all form to cause what we call "gurgle" which is moisture build up. It can also cause a pipe and the tobacco to taste bitter. Different pipes have different requirements, for instance a meerchaum almost never needs any resting.
I try to smoke a different pipe each day and sometimes two different pipes a day. But in between each smoke i'll ream it with a pipe cleaner until it comes out dry and free of residue. I usually smoke between 2-5 bowls a day depending on whether I smoked a cigar that day or how much free time I have. So I guess i'm trying to say, get to know your pipe(s) if it starts to gurgle and taste bitter then rest it for a day or so.