For a QB probably 3 years or so. Look at Eli, he was a late bloomer.
I think you are hitting on a good point about senior years and such. Look at Peyton vs. Ryan Leaf. The senior excells, the junior is a bust. Granted, Leaf would have probably been a bust anyway but still! Also, Matt Ryan vs. Jamarcus Russel. Ryan excells as a senior, then gets off to a remarkable start in the NFL. Russel leaves after his junior year, then wrecks his rookie season with an excessively long hold out. Thus, in a way Russel has 2 seasons less experience than Ryan, when you factor in the lost senior season and the wasted rookie season.