Little while ago, I fininshed a 21k run on coonventional, with 2 quarts top-off. UOA is in that section.
Oil was in passable shape - still within visc. spec, and actually didn't 'look' that bad on the dipstick.
I think as long as you keep an engione topped off, it will go a LONG time on an OCI - these days, maybe even lifetime.
Key is to keep the oil topped off, never let it get very low - I think that a lot of engines that die from lack of oil chages actually die MORE from low oil levels that the actual condition of the oil. I never let mine get more tha 1/2 quart low.