It is balanced around no prestiges meaning that all battles are beatable on normal without any prestige bonuses.
There were also a bunch of people who completed the game before I even made the prestige system, so I don't know.
Admittedly, I have never played from start to finish in one go to time it, but it took me 3 weeks to reach mid-point I believe (on normal), so I expected the overall time to be close to two months.
Did you play in the Idle mode or incremental mode?