The 46 hours is assuming it moves at 300mm/s on that axis for 46 hours, which just isn't the case.
I say this, but as a ballpark figure this is still useful. Even if typical prints probably take longer than that to reach 50km on an axis, that still tells me I certainly don't lubricate them as often as I should.
Maybe that's something printer firmwares could one day be modified to calculate and warn the user about.