At the end of service life, from roll off the line to final shut down, I would put my money on more time on the ground than in the air. Definitely so for aircraft flying shorter stage lengths. Probably closer for the long-haul aircraft that fly 10 hrs at a time then swap crews and go out on another long haul in 2 hours. But I'd still guess that they spend more time shutdown, in mx, and on the gate then they do in the air.