Let's look at some specific ultra-longhaul routes in the world, flying niche aircrafts:

SQ has 5 A345s, running two daily roundtrips SIN-LAX/EWR. So, 4 flights a day, each about 18-19 hours long. That's 74 flight hours spread over 5 planes for about 15 hours each plane, each day in the air.

Thai has 3 A346s, running 4 weekly flights to JFK and 4 weekly flights to LAX. So, in a week it's 16 flights total, 17-18 hour each fligth. That's 280 hours per week, or 40 hours per day for 3 planes, or 13.3 hours each.

AC has 2 A345s, running one route YYZ-HKG daily. So, also, 15hr in the air each plane, each day.

[CX has 3 A346s, mainly for one route JFK-HKG, but they use the extra planes for other routes regularly, so hard to say how much time in the air actually.]

But an aircraft doesn't need maintenance after every roundtrip. Cleaning, catering, and fueling is all it needs. So, yes, a particular aircraft may only need about 2 hours on the ground for 19 hours in the air, for the longest routes like SIN-EWR, if they don't need to run on a regular schedule.