I was running a timer test on the Hero 5 Black, for our upcoming hunting trip.

We put the cameras by waterholes to take a video as long as the card or the battery would allow.

The following results were with fully charge battery in the camera, plus a 10,000 mA external power pack, and a 256GB card.

Set for 4K video, about 8.5 hours.
Set for 1080P video, about 15.50 hours.

If you are planning to do this, my suggestion is to check the power pack, as I have tried some with higher claimed charge but never lasted as long.