It's been 5 or 6 years since I rode the Rockhound so I don't remember exactly, but I think it was an 8-10hr day for me. I stopped to help a guy who had drowned his bike in a water crossing which added to the time, but I also just rode the course and didn't try to hit any of the official waypoint locations/activities for scoring/prizes. I was very tired long before the end of it.
For reference in terms of stamina, at the time I would usually ride around Ganaraska single-track for 4 hours straight at a decent clip, with maybe a 5 minute break in the middle. In terms of difficulty, the Silver Rockhound route was definitely harder than anything in Ganaraska, but nothing extreme from what I recall. No babyhead rock cliff climbs or anything like that, although I did get stuck up to the axles in mud at one point and there were two or three long water crossings with water up over the top of my boots. You do need to be comfortable with steep rooted hill climbs and descents, log crossings higher than axle height, and rocky sections.