Well, stamina for hitters doesn't change much. Which is why you don't know what you're talking about.
I just showed you a pitcher who's stamina improved 22%, which again, isn't far off considering you only have a scouting budget of $14M. Being off by 8% isn't that bad.
As for position players and durability, here you go:
https://www.whatifsports.com/hbd/Pages/Popups/PlayerRatings.aspx?pid=8051646
Bruce Milton's stamina has already improved by 25% in 3 seasons. I suspect it'll be right around 77 (30%) before he's done developing.