The length of time it takes to beat Kena: Bridge Of Spirits really depends on two things. Each "Act" begins with you being given a mask that belongs to a specific character who features prominently in that section of the story, so you should know roughly how far into the game you are from this. Without delving into spoiler territory, Kena: Bridge Of Spirits can be split into roughly three "Acts", each of which introduces a new chunk of the landscape to explore.
Kena: Bridge Of Spirits length: how long to beat the game?