I don't want to leave the calculations to me because well the game did that and it says I rolled through the world's strongest in less than a day. My guess in all seriousness is roughly a month like jman said. The first few events seem to line up as if they happened moment after moment because the player is dragged into it. Future events imo can be seen as the player taking an active role in helping. So with losses against gym leaders and especially ones that want to kill him like Corey, the player takes time to make sure they wont die. Or if you decide to become a student at Kiki's school and train with students, (if you choose to see it like me) then the story can be kind of lengthy which is nice. Because otherwise like Damaino said, you would be unnaturally strong which Reborn as a whole has proven that no one was born with talent, that is earned here. Didn't think I'd get exacts more curious as to people's thoughts,