You can only make some sort of educated guess.
cruising at a steady 6000rpm should be fine forever, 7,500rpm should be OK for tens of thousands of miles
It's fine to run it to red line (and beyond
) when accelerating, but, not to hold it there
Problem is, not sure if it's a cumulative effect?
Bits wear a little, metal particles get embedded, oil pressure drops and cam seizes, or, high rpm, oil pressure drops to cam, and rockers overheat, cause oil breakdown, then bearings 'pick up' or seize
I couldn't really say a definite rpm as some motors seem to run fine at higher rpm (although I haven't stripped one recently)
I would say sustained 9,000 or more rpm and something bad will happen, but, it still takes several miles and does seem to be cumulative (cam is done around 130~170 miles of 5th/6th gear running, even with pulling clutch and allowing rpm to drop every 3~5 miles)
It was never designed to be beat up and down interstates or motorways (although I knew several people who did it, me included
)
At present, the 'secret weapon' is Shell Rotella 5w/40 full synthetic
Idle speed probably needs to be slightly higher than normal to keep oil pressure up at idle (have mine set around 14~1500)
I'm still working/playing with remote oil line to head and drilled cam, probably the most cost effective way to go, pretty sure just about all 360's are eventually going to 'eat' the top end eventually (except for a few modified ones )
The worst thing you can do is use thicker oil (40 wt, 20w/50, etc)