Let's take a look at Pass Attempts Per Game (APG) and Pass Yards Per Attempt (YPA) for Bama QBs going back to 2011 when AJ first started.

2011: AJ McCarron - 25.2 APG (8.0 YPA)
2012: AJ McCarron - 22.4 APG (9.3 YPA) LED FBS IN PASSER RATING
2013: AJ McCarron - 25.8 APG (9.1 YPA)
2014: Blake Sims - 27.9 APG (8.9 YPA)
2015: Jake Coker - 26.2 APG (7.9 YPA)
2016: Jalen Hurts - 25.5 APG (7.3 YPA)
2017: Jalen Hurts - 18.1 APG (8.2 YPA)

All first year starter's since AJ McCarron has gotten at least 25.2 Attempts Per Game. I think it's entirely possible Tua gets a few more than that, with a ceiling of 30 APG. So we'll look at his "floor" (he could always get less, but we'll consider this his floor for this purpose) and his "ceiling". 25 APG and 30 APG respectively.

As for his YPA, I think it's fair to expect somewhere between 8.5-9.5 considering the insane number of weapons and Tua's throwing ability. I'm going to use 9 YPA to meet in the middle for these purposes.

Tua's Passing Yardage with 25 APG and 9 YPA
Through 12 Games: 2,700 Yards
Through 13 Games: 2,925 Yards
Through 14 Games: 3,150 Yards
Through 15 Games: 3,375 Yards

Tua's Passing Yardage with 30 APG and 9 YPA
Through 12 Games: 3,240 Yards
Through 13 Games: 3,510 Yards
Through 14 Games: 3,780 Yards
Through 15 Games: 4,050 Yards

Now, I don't think he'll have to throw it 30 times a game, I'm just saying that's the high end of what may happen in terms of attempts. He could have less attempts and less YPA for all we know, or less attempts and more YPA.