Keep in mind MC, the other two are okay with it because polygamy is normal to them. It ain’t normal to the hero’s descendant. Finding out that he has 2 other lovers is gonna cause a bit of drama.
They started banging him after he fully mastered magic and qi-gong.
So probably not hundreds of years and more like many weeks or months, since it's unlikely that he left immediately and I don't think the demon and dragon king would only do it with him once (MC seems a little bit...too practiced in this chapter for me to think otherwise).