The A. Lange & Söhne Lange 31 stores its 744-hour power reserve in two mainsprings. Roughly how long is each one?

The story behind it

Each of the two mainsprings is 1,850 mm long, about ten times a normal watch mainspring. At that length the torque is so high and so uneven that Lange added a patented constant-force escapement, so the balance gets the same impulse on day 30 as on day one. Winding by crown was not practical either: the watch uses a key that goes into a square socket in the sapphire caseback. It was also the first mechanical wristwatch with a 31-day power reserve.