Depends on the engine...
Worn engine with blow-by, it is thickening of oil and TBN depletion due to trying to neutralize acid formation.
New engine, it is either shear, or AW depletion. But the later can be tricky because AW compounds form over time and are unique the engine. So as long as the TBN remains tolerable and the oil is not thickening, it can go a long ways. We have a member here with 62,000 miles on original fill of Amsoil with by-pass filtration and top-ups when he changes filters.
Alcohol and GDI engines, or old carb'd engines out of spec can have fuel dilution and that is a real issue ...