now, if you're asking how long can you ride wide-open at a stretch:
i ran WOT 20-25mph (i had 60lbs of supplies in my surplus ALICE pack) for an hour at a stretch, then stopped to let it completely cool, give it a 10mm wrench-test, & top the tank...i covered 165 miles of coastal mountains in 12 hours.
she's still running great right up to this very mile 8)
You burning castor oil in your fuel mix? You are proving what I have been saying about these engines: They work fine if ridden hard and long provided that they are broken in and run with a good quality oil.
I use bel-ray mc+1 synthetic 32:1 Here in tucson we have a 20mph limit, what rpm that is I dont know but I did a 42mile trip none stop. And that was at 110 degree's. Reading you guy's, now I wonder if I shouldnt pull it apart and make sure I have not damage anything.