The math in the last post works, but ONLY if the oil is lost due to leakage, NOT by worn cylinders or rings.

If the engine were worn such that was BURNING the oil due to blowby conditions, you actually should change oil MORE often. This is because there is a serious amount of combustion byproduct contamination making its way past the rings and into the crankcase as well as through the crankcase vent.

Good luck,