Right, there might be some brain twister in here that I overlooked in terms of working out times in the air, but I decided to compare the Norwegian schedule to BA G-BZJB - their oldest Dreamliner, courtesy of BA Source.
Code:
From To Dep Arr
LHR EWR Wed, 18:45 Wed, 19:00
EWR LHR Wed, 23:00 Thu, 10:25
LHR YYZ Thu, 13:35 Thu, 14:45
YYZ LHR Thu, 18:45 Fri, 06:15
LHR YYZ Fri, 13:00 Fri, 15:00
YYZ LHR Fri, 18:45 Sat, 06:30
LHR EWR Sat, 18:45 Sat, 20:30
EWR LHR Sat, 23:00 Sun, 09:45
LHR HYD Sun, 14:35 Mon, 05:05
HYD LHR Mon, 06:45 Mon, 12:55
LHR EWR Mon, 18:45 Mon, 21:00
EWR LHR Mon, 23:00 Tue, 10:25
LHR HYD Tue, 14:35 Wed, 5:15
HYD LHR Wed, 06:45 Wed, 12:55
Now this is not as agressive, and there's a curious 12:15 park at LHR on the weekend in there, but there are times where the plane is in the air for more than 17 hours a day.
I still don't think Norwegian's schedule is unachievable, but as pointed out it might not be "on time" in a lot of cases. After every JFK arrival there is 4:35 to 5:25 to recover at Gatwick which should be enough - until something goes completely wrong. That's where BA's park at Heathrow would come in handy...