How to measure for how long a window is active?

I am a digital artist, using Clip Studio Paint (a Windows app) to draw, through Wine. It would be interesting to know how many hours each piece takes me. But being how I am, most of the time I just forget to set up a chronometer to measure or to stop it after I end or during a break.

Would it be possible to have a script that logs how much time a window is active, so I can add that up and get a time?

EDIT: When I talk about the window being active, I specifically talk about the window being in focus, not just the program running, so that if I get distracted and alt-tab out I can get later a closer idea of how much I actually worked.