that it comes down to body type, leg-strength, and delivery of the pitcher.
You can tell when a pitcher is getting fatigued and needs to come out. All you have to do watch their delivery and the location of the pitch. Is their delivery changing? Are they not finishing their delivery like they were two innings earlier? Is the leg drive toward the plate still there?
Location- is the ball starting to stay up? A tired pitcher wont bend all the way down on his follow thru- thus leaving the ball up more. When this starts happening- your pitcher is tired.
I had a pro prospect pitching for me as a 10th grader. His Dad stuck his nose into what we were doing way too much because he was so sure this kid was going to the bigtime. I had him lifting weights to build his skinny frame (6'4, 160), running after he pitched...he didnt throw alot of breaking balls- FB was 87-88 and he had a great change that had some movement. He helped lead us to the State finals where we lost to a team with a 2nd round draft pick on the mound. But the Dad was livid I let him throw 134 pitches in our final regular season game against our rival. He didnt pitch for another 9 days afterward, so I extended him a little in a big game, and watched him closely in the 6th and 7th.
The Dad decides his boy needed to transfer schools so that the self-proclaimed "pitching guru" can guide his career. This coach didnt make him lift weights at all. The kid goes off pitching all summer and Fall with travel teams, then in his 3rd start of his Jr season at his new school- tendon pops and it's Tommy John time. The kid doesnt know the meaning of hardwork to rehab it because Daddy had always told him he didnt need to lift weights or anything. He pitched in college, but was never the same pitcher- and didnt get drafted.
If kids are in a good program that manages their arms and builds their bodies to help also- then they can go into the 130's. Do you do it every start? Hell no. But it doesnt hurt from time to time in a big game if you make them run, lift, take the anti-inflammatory's necessary to keep them and their arm in top shape