Ancient Chinese Saying #437: ... 10,000 days for the sword
Conceptually, this is not that new. The Tang-dynasty poet Jia Dao (779－843) had a famous poem starting with this phrase:


  十年磨一劍 "For ten years I have been polishing this sword"  


The "polishing" here is a poetic expression of his learning swordcraft. The phrase could be even shorter as 十年劍 ("ten years of sword") in Chinese is also similar to "10,000 days of sword" in stucture.
The original text from The Legend of the Condor Heroes was an explanatory description of a fighting phase-change between charaters. You could find the exact phrase online as quoted below from Chapter 20 ("The Altered Manual") in Frans Soetomo's translation:


  There is a saying among martial arts practitioners, “A hundred days to master a saber, a thousand days to master a spear, ten thousand days to master a sword,” indicating that sword techniques were the most difficult to learn.