I've done 1200* for 3 hours no issues at highway speeds. I just can't find a clear answer anywhere on what's the highest safe sustainable temperature I can run at which won't cause any issues down the line.
That's because Cummins doesn't publish EGT numbers. You won't find concrete numbers anywhere.
In my opinion, 1200* is the highest continuous temperature I will run. I've run 1200-1300* (trying to keep it at 1200) for as long as 8 hours towing on the highway before, no problems. During some of those tows, I got distracted and let the motor hit 1500* EGT. When I noticed how hot it was, I jumped right out of the throttle, no side effects. Motor still runs like a scalded dog.