I might have to bring it up to a minute or two before I'd give you that -- I perceive the exponential growth in disutility for extreme pain over time during the first few minutes/hours/days as very, very steep. Now, if we posit that the people involved are immortal, that would change the equation quite a bit, because fifty years isn't proportionally that much more than fifty seconds in a life that lasts for billions of years; but assuming the present human lifespan, fifty years is the bulk of a person's life. What duration of torture qualifies as a literal fate worse than (immediate) death, for a human with a life expectancy of eighty years? I'll posit that it's more than five years and less than fifty, but beyond that I wouldn't care to try to choose.
Let's step away from outright torture and look at something different: solitary confinement. How long does a person have to be locked in a room against his or her will before it rises to a level that would have a non-zero disutility you could multiply by 3^^^3 to get a higher disutility than that of a single person (with a typical, present-day human lifespan) locked up that way for fifty years? I'm thinking, off the top of my head, that non-zero disutility on that scale would arise somewhere between 12 and 24 hours.
"What's the worst that can happen?" goes the optimistic saying. It's probably a bad question to ask anyone with a creative imagination. Let's consider the problem on an individual level: it's not really the worst that can happen, but would nonetheless be fairly bad, if you were horribly tortured for a number of years. This is one of the worse things that can realistically happen to one person in today's world.
What's the least bad, bad thing that can happen? Well, suppose a dust speck floated into your eye and irritated it just a little, for a fraction of a second, barely enough to make you notice before you blink and wipe away the dust speck.
For our next ingredient, we need a large number. Let's use 3^^^3, written in Knuth's up-arrow notation:
3^^^3 is an exponential tower of 3s which is 7,625,597,484,987 layers tall. You start with 1; raise 3 to the power of 1 to get 3; raise 3 to the power of 3 to get 27; raise 3 to the power of 27 to get 7625597484987; raise 3 to the power of 7625597484987 to get a number much larger than the number of atoms in the universe, but which could still be written down in base 10, on 100 square kilometers of paper; then raise 3 to that power; and continue until you've exponentiated 7625597484987 times. That's 3^^^3. It's the smallest simple inconceivably huge number I know.
Now here's the moral dilemma. If neither event is going to happen to you personally, but you still had to choose one or the other:
Would you prefer that one person be horribly tortured for fifty years without hope or rest, or that 3^^^3 people get dust specks in their eyes?
I think the answer is obvious. How about you?