I assume you mean written off, and the answer is it's generally not about how long, it's about how many "cycles". Plane maintenance is usually required on a sch

I assume you mean written off, and the answer is it's generally not about how long, it's about how many "cycles". Plane maintenance is usually required on a schedule based on how many flight hours or cycles, and it's usually the cycles they hit first. A cycle is a pressurization and de-pressurization of the cabin. This is what eventually kills a jetliner, the hull will fracture from the stress. The engines are designed to rebuilt/replaced periodically. The hull cannot be reasonably repaired once damaged. Airliners are required to do different checks at different stages, and the big one costs millions of dollars and requires them to basically disassemble and rebuild the plane. They even may have to remove the paint to check the hull for issues. Industrialized nations generally will only do this a couple times before the cost is more than the jet is worth (I think this is generally around 20,000 cycles, depending on the plane) then they sell them to a third world airline who doesn't have as strict a regulatory scheme. Even then, they generally live on for quite awhile, our regulatory scheme is designed to get as close to zero risk as possible, not to be economical, and it's hard to argue with the results. This is one of many reasons why there are an order of magnitude more crashes in the developing world, though even still safety is high relative to almost anything other than first world commercial airlines.