That probably depends on the size of the dungeon, how much can be skipped, what else is in there and how hard it will be in the end. With 855-860 loot, that would put it to Mythic+6-9 for baseline loot. But I highly doubt it will be that hard.
The only reference so far I have was the fatboss video that shows all bosses. They said their first run took over 3 hours (but that was with scaled itemlevel).
Edit:
Here is the video:
https://www.youtube.com/watch?v=tHK2pbrNU38
They do say they think if you know what to do and have good gear (as in, really good raid gear) it will probably "only" take 1 - 1 1/2 hours to clear the whole dungeon.