Back in 2011 I took a look at 42 past BYU games for which I had at least partial information about length of the pass (relative to LoS). For those games, these were the completion percentages on passes over 10 yards:

57% 11-12 (147 attempts)
51% 13-15 (146 attempts)
47% 16-18 (119 attempts)
41% 19-21 (117 attempts)
34% 22-26 (101 attempts)
30% 27-35 (109 attempts)
28% 36+ (102 attempts)

That's the percentage actually caught, not the percentage of "well-placed" attempts. Here's the percentage of long throws that I considered completely uncatchable:

19% 11-12
21% 13-15
28% 16-18
28% 19-21
39% 22-26
39% 27-35
32% 36+

The remainder of the throws are drops, non-drops that the player touched (typically off the hands -- poorly thrown but potentially catchable), and pass breakups or interceptions (includes some good throws and some bad throws). Drops increase as a proportion with distance, and throwing drops into the mix I get this percentage of good passes:

60% 11-12 (147 attempts)
56% 13-15 (146 attempts)
50% 16-18 (119 attempts)
46% 19-21 (117 attempts)
38% 22-26 (101 attempts)
37% 27-35 (109 attempts)
35% 36+ (102 attempts)

IOW if only half of Taysom Hill's long passes are "good", that may actually be a fairly typical performance.