Depends on the trail and how fast you want to go. I've ridden some loops that have been 95% off road in the Lakes on a 80mm XT hardtail, because they involved a long slow drag to the top of the pass, a good few miles of seriously tech but not slow-ish singletrack and a wide open decent which gave plent of time to pick the smooth lines.
Other tracks I've been on 130mm hardtails hanging on for dear life as the fork/my body cant cope with the repeated big hits. Cant name the tracks, but one, I suspect was a footpath, through a disused quarry north west of Conniston heading south, the other was the rootiest, rockiest, twistiest trail I've ever ridden from the top of Grizedale forrest to the shore.
One ride perfectly suited a stiff 80mm fork, the other would have been fetter on an even bigger bike (although it would have made the other 28 miles of undulating singletrack and big climbs utter hell).