Typical RTR engines last up to 4-5 gallons max with modest care and abuse. Expect the same from an OS, honestly. Maybe more with better care - good fuel and carb setting is paramount. Some guys report 12+ gallons from the Italian Engines - Picco and Nova and their re-brands. Nova is more affordable than ever before - AND they’re stupid powerful and easy to operate.

So as said - quality of the engine will play a role in the lifespan as does the care and operation.