Entire life of the engine. After 1800 hours total operation, the engine should be overhauled: taken apart, cleaned, each part examined and replaced if necessary. Note that per FAA regulations in the USA, only actual flight time counts toward this limit, not time spent taxiing on the ground.

It'll take a normal private pilot a few years to reach 1800h of operation, so it's more economical if you're not flying the plane continuously for weeks at a time.

But yeah, it's expensive. Owning a plane is expensive. It depends on if it's worth it to you to operate the plane. Small single-engine planes like the Cessna 172 are commonly used for recreation or training. So if you use it often, such frequent overhauls will pay for themselves in terms of safety and reliability.

Maybe when all-electric recreational aircraft become common they won't have to be maintained quite so much...