Meh. I think it's less about workload than monitoring fatigue, he's not some skinny 19 year old, between genes and a good conditioning program he's probably physically ahead of many 22-23 year olds.
It's more important to focus on his mechanics, signs of fatigue, and avoiding overwork in any one game where he develops bad mechanics due to fatigue. I think if handled carefully he could pitch 200 innings, as long as you don't let him pitch tired - you need experience to know how to adjust when you arm is tired so to avoid excess stress on your shoulder, etc.
If he maintains good mechanics, a lot of the power comes from his legs, which is why a well designed off-season conditioning program is important - and that's why the Phillies have invested money into both trainers and video to analyze bio-mechanics.
Painter has command of four pitches, he's not some kid who basically has a plus FB and little else and will try just to blow it by every hitter. And has an experienced catcher who can tell when he's falling into that rut, and force him to use his secondary pitches.