The key complication is that it's only worth running flights when people want to fly. Essentially this means that, overnight, a good proportion of the fleet will be sitting idle. There's only so much longish eastbound business to be done and the rest will all be overnighting at the airport so that they are ready for an early morning departure. After that, then top efficiency would have operations from say 6am to 10pm (16 hours) minus several 2 hour turns during the day. So 8 hours utilization for a narrowbody would be pretty well as good as it gets.

Widebodies can be much more heavily utilized. Take for example EWR, FRA and SFO. You could devote two planes to this route. Plane 1 would start the day in FRA and fly to SFO (c 12 hours) and then fly back to FRA (c11 hours) arriving at FRA the next day so that it's ready for its next flight about 3 hours later than it left the previous day. Plane 2 would start the day 3 hours later than Plane 1 and would fly to EWR (c9 hours) and return overnight (c8 hours), ready for a departure again some 3 hours earlier. They would average about 20 hours in the air each.

This would be about as optimized as you can get. Most routes cannot sustain that sort of timetable.