How long should a successful rebuild take?

Interestingly you left out ‘injuries’. Nothing can derail a rebuild quicker than having a series of key injuries that keep your playmakers on the sidelines for much of the season. While all teams have injuries, sometimes these become so overwhelming that it is impossible for a team to make progress during a rebuild.
I get what your saying to some extent but if you draft a Allen/Mahomes level QB, even losing them for one season put's you miles ahead of must teams. I would rather be the team with an elite QB, great coach and roster that has a bad injury year vs. a healthy team that's filled with mediocrity.

The fact that it's often referred to as injury luck, kind of just exemplifies that main point I was trying to make, landing a SB caliber team just requires a lot of luck.