Despite reports suggesting Kingdom Hearts 3 will have a campaign of around 40-50 hours, and additional content bolstering that number to an estimated 80 hours, Square Enix has not yet actually settled on the final length of the anticipated sequel.
A preview report from Geek.com of Kingdom Hearts 3 published earlier this month suggested those play time estimations had been confirmed. That would make for one of the longest main campaigns in the Kingdom Hearts franchise, and the news of this lengthy play time was reported by a number of outlets as confirmation of KH3’s official length earlier this week.
Source: IGN.com No, Kingdom Hearts 3's Play Length Has Not Been Confirmed