Even then, Peyton and Eli Manning had their regressive years in their 4th seasons.
Three years just like any other rookie. If he doesn't show improvement by then, we move on.
I mean if Cam actually regresses further next season then maybe you start thinking about other options, but if he has a season like this season then you continue to develop him into his fourth year and maybe bring in some competition at that point.
And although he isn't playing elite level QB yet, he isn't playing like absolute garbage either. He is still likely to have 20+ total TDs (passing and rushing) and complete @60% of his passes. Truth be told he has just been pretty average this year. But IMO he hasn't been bad enough to start doing a bunch of had wringing over. YET.