Leigh Steinberg is old school, he was a high powered agent in the 1980s and 1990s, but he wrote a good article on franchise quarterbacks. He doesn't use recent examples, but the advice is still sound. From the National Football Post:
"As a young quarterback, you perform mostly on arm strength and escapability. It takes years to read the field correctly. Looking at the tape of decision making in your early years is depressing." Neil Lomax said to me, "Every year your decision making improves, but the irony is that your head and leadership hit their high points just about when your knees are gone."
The rookie years for promising quarterbacks often involve unfortunate interceptions, lining up over the guard instead of the center and lack of effective time management. It just takes time to develop field mastery. But the limitations of the salary cap take away this nurturing time. Quarterbacks like Steve McNair, Carson Palmer, Philip Rivers and Aaron Rogers greatly benefitted from time spent learning behind veteran quarterbacks.
Yes, he spelled Aaron Rodgers's name wrong. The mentoring Rodgers received from Brett Favre isn't a topic of conversation that either of them open up about, but I got the sense that it really didn't happen. Rodgers certainly learned by watching him, and he was certainly more ready to start for the first time in 2008 then if he was forced into the role back in 2005. But I've never thought he "greatly benefited" from having Favre around for his first couple seasons. I can't imagine Favre being interested in grooming anyone for his job.