I'm reading the manga and I like it. Love that the protagonist is dating both twins instead of being indecisive or having one of them just acting as a wingman (or even worse, a siscon antagonist). Would be fine reading the novel.
Even if we assume that the average lifespan in their other world is 50 years, that would be 500 billion years. That’s thirty-six times the age of the universe.
How the hell could he retain memories of all that and stay sane?