All told, it really is a dice roll of what a first-time starter is going to look like. On average, a typical stat-line is 20/31 for 180 yards, with a touchdown pass, an interception, and 2 sacks against, but clearly there is significant upside for a great game, and significant downside for a bad game. Normally, though, the first-time starter has a below-average game.
In fact, the above "worst games" exclude low pass attempt games, including some hilarious ones. Saskatchewan's Seth Doege once went 3/9 with 3 picks and a sack before he was yanked for the aforementioned Tino Sunseri, who went on to throw for 274 yards and a TD.
Montreal fill-in QB Josh Neiswander went 1/3 for 3 yards and an interception and then was replaced by Tanner Marsh, who came in to throw for 329 yards on 14/32 passing and 4 INTs. Oh, and somehow, Montreal
ended up winning the game, after a 57 yard pass with 9 seconds left set up the game winning field goal.