In ch 1, his sister login countdown was 252 hrs.
In ch 5, he said that her login countdown was 2 hrs left when he finished his tutorial. That means he spent ~250 hours leveling up in the tutorial. (He chose this because he can revive in tutorial but not in the actual game.)
In the same page in ch 5, he also said that his sister countdown became 10 months 23 days in game time. That means the time flow is slower in the game, with 1 hr tutorial = 3876 hrs in game.
That means in the ~250 hours he spent in the tutorial, ~969k hours or ~112 years have passed in the game.
The other players probably only spent couple hours in the tutorial, that's why they have been in the game for about 100 years.
Edit: actually I just remembered that the players got to keep their account, so the others might not even need the tutorial. MC's account was deleted, so he started fresh from the tutorial when he got summoned.
