For info
Flew this evening 3hr 20 mins at FL 390 737-700.
Started with 11 tonnes, finished with 3.2 tonnes.
Hefty layer of ice underneath and on top of wing at destination (+12 C).
After refuel for return (11 tonnes again) took 35 mins for upper surface ice to dissipate.
Slight delay to departure saved a fortune (and probable further delay) in de-icing bill!
It's just a fact with the -700!