I'll start off by saying what most people in the know, people who have actually played and coached, say (heard repeatedly on the NFL Network):
" You don't learn by watching; you learn by doing"
All those who pretend like they know, BUT DON'T, go on about the sit and learn routine. Aaron Rodgers is one of the few that sat. He didn't do it for his own good, he did it because he was forced to sit behind a HOF QB. To say that he would not have been good if he started playing earlier is just ludicrous. You have to do and learn on-the-job; the longer you wait, the longer it takes to get you up to speed.
So all you "experts", trying to sound soooo wise with the "sit and learn" qualifiers . . . pfffft!
That said: You do have to be a MENTALLY TOUGH, confident player to start as a rookie QB in the NFL. Those qualities are the most important components of a QB's DNA. Everything else you can learn; that component of DNA you're born with. Alex has everything else, but he lacks that. That's why he'll never be a consistent championship winning QB; but he will be a serviceable back-up.
If Colin has that component of DNA, and I believe that he does, and if Harbaugh is true to his word that the QB competition is open and will remain so for the season, then I'll go out on a limb and predict that
Colin will be starting by the end of the first year. All the best QB's in the league have started out that way, unless they were behind a bone fide starter. Rodgers, behind Favre. Brady, behind Bledsoe(but only for a year). Rivers behind Brees.