Depends on how fast you go. Today I did a 102km ride with 2151m of climbing (about 63 miles with 7000 ft of climbing). There were 4 cols, each was about a cat 3. That is, about 400m (1300ft) of elevation gain over 6 or 7km (4 miles), for an average gradient 6%. Average speed going up was 18 kmph (11 mph), and was relatively easy. I did the ride before lunch and didn't bother with bringing any food, other than one of my water bottles was a 50% mix of water and orange juice.

A cat 1 is harder, of course. About 800 to 1500m of elevation gain with an average gradient of around 8%. HC generally has 1500+ meters of climbing with lots of sections above 10% (by "section", I mean at least a 1km stretch, not some short pitch of road). On these climbs, I eventually get cooked. 10km of climbing at 10% grade is MUCH harder than ten 100m rollers even if they have the same gradient. It's the lack of any rest that eventually wears you down.