This isn’t the intended purpose for this app, but it can sorta work for your needs.

https://play.google.com/store/apps/details?id=vikesh.dass.lockmeout

You can set up the thing to lock on a schedule for one minute. If you don’t mind being locked out for a minute occasionally while you’re using your phone, it effectively means that if enough time passes your phone will have locked and you’ll need to use your passphrase.

Edit: anyway the existence of this function means it’s possible to write an app that does what you want.