Well known is the last long endurance test flight by a Boeing B787-8, which drew its shape in the air for 18 hours while on a long endurance test flight: https://www.wired.com/story/boeing-787-8-drawing-test-flight/.
What exactly is the purpose of these long endurance test flights? What is tested, what shall be proven? - surely not that the aircraft can actually stay in the air that long, because that test flight happened long after the introduction of this type into the market...
On a side not, does every commercial airline type have to undergo such an endurance test flight?