Welcome d_rock. The following may not answer your question, but I like the sound of my own voice...
I would guess that Lefty Grove '39 could go far deeper into a game than say Phil Niekro '67 even though their IP/162 are equivalent. Phil would struggle to get a CG as his IP/G is low compared to Lefty's 8+ IP/G. Even though real life Phil had CGs, he also appeared in relief, so WIS Phil will have limited stamina in a single start.
I'd sub out a starter at around 80% fatigue in a live game, though occasionally a P can have success even below 50%.
(Sound of my own voice? That makes no sense. Oh yeah? Says who?)