There is no"one size fits all" advice possible. It depends on the way the pipe's made. And how sensative you are to taste quality.
The issue is headspace. That's the gap (if any) (and there usually is) between the end of the tennon (the round tube at the end of the stem that goes inside the briar) and the face of the mortise milled to receive it. If there isn't a flush fit here, you've got a condensation chamber inside the shank of your pipe.
Curiously perhaps, this isn't a bad thing as far as taste goes. But the smoke that's swirling around in there (due to the diameter change in the airway) is condensing tars. This is where the problem is you're inquiring about.
The next time you've finished smoking your pipe, after maybe ten minutes or so, twist the stem out. (IT DOESN"T MATTER WHEN YOU REMOVE THE STEM. IT DOES MATTER WHEN YOU PUT IT BACK IN AGAIN).
Fold a kleenex over on itself about five times (the max possible), then double it over on itself. Twisting this, work it into the shank, and keep twisting it in until there's no more room for it.
When you've pulled it out, you can see how much condensed gunk was in there. (Also how much of it was condensed on the face of the tennon). With some pipes, this is hardly any. With others, it's a lot.
When this stuff never has a chance to dry out completely, it starts to ferment and mildew, and you've got an increasingly substandard taste, and finally a sour-tasting pipe. The solution for this isn't buying some patent pipe sweetener -- it's either giving it a long time to dry out inside between smokes through evaporation (with the stem in, there's not much air circulation to help matters any) OR just keeping the sucker clean in there (see above) and leaving the stem out 'til the next day (so the air can get to it and finish the job). Doing it this way gets it ready to go again at optimum quality in the shortest feasible time.
When the stem will be ready to go back in again depends on the pipe, and how many times it's been smoked in succession. If it still feels tight, give it more time to settle back down.
How often you do this -- again -- depends on the pipe. Keep it clean and it will keep you happy.
:face: