This achievement is in the epilogue/ end game, and is achieved when you get to the end of an 8 hour restore/ reinstall progress bar/ screen.
So when you're on the end screen where you click x to continually hear and see Super Hot over and over again, this is not where you leave your Xbox.
You need to activate one final node (white rectangle) to trigger an 8 hour timer. You find this node by following a thin white line activating a few nodes along the way. You know when you've activated the final node as you can see a physical timer counting down from 8 hours. I can't remember the exact directions for this but once your on the white line its hard to miss.
If you close the app/ game I'm pretty sure the timer will continue where you left it, if someone finds anything conflicting to this I will update solution.
Make sure you change your power saving settings so your Xbox doesn't turn off. I also plugged my pad into the Xbox using USB to make sure my controller didn't turn off. Not sure if this affects the timer or not.
If anyone thinks anything needs adding to this solution just let me know has this is my first solution.
Edit#1 - Thank you to the few people who have confirmed that the timer is now 2.5 hours instead of 8 hours.