No real rule of thumb on number of chamberings, Jason, because that will vary according to gun and cartridge. If the grasp of the case mouth on the bullet is tenuous, a single chambering might do it, particularly if the gun in question doesn't have a very straight-line feed. Some securely crimped rounds can take several cyclings without measurably changing dimensions.
A better rule of thumb is to measure each such round against a factory-fresh round from the same box or lot. As soon as the round in question is found to be visibly shorter, it's time for it to be consigned to the practice pile and replaced with a fresh cartridge.
This may seem overcautious, but bear in mind that if the projectile has moved rearward at all in the casing, the seal has been broken and it may no longer be moisture proof, even if it's not yet far enough back to cause over-pressure issues. Remember, too, that each trip up the feed ramp and each violent pull/push from extractor/ejector can put more tiny burrs on the case, potentially compromising its feed reliability the next time. The chance is small...but so is the replacement cost, and this is the ammunition on which our lives and the lives those we most love may one day depend.
Just another guy's belt-and-suspenders opinion.