I keep seeing people saying they have played xx hours or xxx hours so far. Is there somewhere that this is recorded by the game or are people just making estimates?

Steam will tell you on the game's library splash page. No idea for gog or stadia users.

Gog tells too but have to click on the game pic in the lib

Thank you lady and gentleman.

GOG Galaxy gives you the time played as well, including monthly/weekly/daily totals.

Ok let's do this: 120 hours last time i checked. What's yours?