Most of the great QBs were at least good by year 2 as a starter.
You can go all the way back to the early 90s. Favre got his first starting job and was great right away. So was Roethlisberger. Rodgers sat for 3 years, but as soon as he got his chance, he was great. Peyton Manning, McNabb, and Culpepper(before injuries wrecked his career) popped by year 2 as a starter. Brady was above average immediately, and developed into great. I think people mistake being above average immediately and developing into great for being bad and developing into great. Russell Wilson was above average the moment he stepped on the field as well. So were other good QBs like Romo and Rivers.
Going back further, Marino was great right away and so was Jim Kelly. Joe Montana won his first Superbowl in his 3rd season, and it was his first full season as a starter. John Elway won 12 games in year 2. Matt Stafford got injured his second year and played only 3 games. In his first full year as starter threw 41 TD passes.
If I remember correctly, average as hell Joe Flacco and Kirk Cousins were at the very least competent as soon as they started games.
What this tells me is that the good and great ones reveal themselves basically as soon as they take the field.