Tom Brady sat one year. Granted, with him things were sped up because of an injury to Bledsoe, but ultimately he only sat on the bench for one season. And no, these days it is not common for QBs to sit behind veterans for several years. It was that way In the 1980's and before, and probably in the 1990's to a lesser degree, but these days that is not how it works the majority of the time.
Don't get me wrong, I'm not saying Dak isn't going to get better with the added years of experience, but it seems you are treating him differently than other QBs in this era, assuming many others got to benefit from several years of sitting and learning before stepping into the starting role. That really isn't the case. That's rare.