I agree with
@BraveHeartedHero. Furthermore I'd say: yes 100 Million Years (or rather 200 Million) is far too unnecessarily long a time span for various reasons, except maybe for he'd be way more OP.
I'd argue that he alone would not be able to master swordsmanship
technique or any other non-physical feat (except maybe mindfulness and similar) by himself without someone (or something like the Internet)
teaching him.
But bottom line for me is the 100 Million years are probably derived from a Chinese saying or tale, rather than being an [in]accurate depiction of reality and thus don't matter too much for me in the story.
One of the aspects of why I personally enjoy this manga over others is that it,
up until now at least, actually builds on the master slave dynamic and has (IMO) a better than average reasoning as to not going back to the normal.
Where as for example the anime "The Asterisk War" basically completely ignored the master/slave dynamic after the initial contracting.
Anyway I want to end this comment by saying that I write this all in good will and I (try 🙃) to respect everyone's differing opinion and look forward to all the manga/anime suggestions of anime/manga which are better than this one (and share the same concept)
This doesn't have to the case, I like me some good (different) manga ![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
as this manga.
P.S: This may look way convoluted, but there's no way for me to tell before actually posting
.