EDIT: It seems that, at lease for the PS4 version, 4 hours of real life play session equates to one day in the in-game world. So you could easily find how many days have elapsed within the game world, multiple by four, and get how many hours you have spent in game. This should apply to PC as well

Courtesy of @MartinEnder for finding that out!

So what can I do?

The only possible ways are:

• Looking at the time in-game, and calculate the offset you need to multiply to get the real time. An easy way is to play 10 minutes IRL, and see how much time passed in-game. This will give you the number you need to multiple to get your answer
• Look at the Windows Logs (assuming you are using windows) and see when you opened and terminated (close) the *.exe. If you use a Mac, then you would need the appropriate logs Mac uses to track the change in programs. If you don't know how to access the logs:
  • Ask on the Super User Stack Exchange page. They should be of great help.
    DO NOT ASK HERE, IT IS IRRELEVANT FOR GAMING AND WILL BE CLOSED AS SUCH
  • Use your search provider, should give the same answers

Recomendations

I would recommend you to add the game on Steam, even as a non-steam game. This will allow Steam to still track how long you have played, without you needing to dig into the logs