In Windows, head over to this folder:

C:\Users\User\AppData\Local\FortniteGame\Saved\Config\WindowsClient  

and find a file named "GameUserSettings"


Open up GameUserSettings file:

step-1

You'll see all of this text:

step-2

Scroll down until you find "SimpleMobileStats"

step-3

Under SimpleMobileStats you will see "SecondsPlayed=#######"

Copy the numbers after SecondsPlayed= and go to Google.com. Paste the numbers and type "seconds to hours":

step-4

Now you know how many hours you have spent playing Fortnite!