About 80 hours according to Jonathan Blow.

https://en.wikipedia.org/wiki/The_Witness_(2016_video_game)

Personally I never completed it but I still enjoyed the parts of the game that I did do. So it's not like you have to spend that many hours on a game, you can quit at any time. And on the flip side if you do want to put many hours into it there are a lot of puzzles to be explored.

I personally completed the main game in about 20 hours, but there’s lots of optional stuff if you want to keep going.