If the main reason a small amount of torture is much worse than we might naively expect is that even small amounts of torture leave lasting, severe psychological damage, should we expect the disutility of torture to level off after a few (days/months/years)?

In other words, is there much difference between torturing one person for half an hour followed by weeks of moderate pain for that person and torturing one person for the same amount of weeks? The kind of difference that would justify denying, say, hundreds of people a fun weekend where they all learn to waterski?

I'm not sure what exactly you're getting at with that specific example. I think that yes, torturing someone for weeks, followed by years of psychological pain is significantly worse than torturing someone for half an hour followed by weeks of (probably a bit less severe) psychological pain.

Your general point, however, I think definitely has some merit. Personally, I wouldn't expect to see much psychological difference between an individual who was tortured for five years versus a person who was tortured for ten. I would definitely expect to see a larger difference between someone tortured for six years versus someone tortured for one. Certainly there's a massive difference between 5 years and 0. There probably is some sort of leveling off factor. I don't know exactly where it is, or what that graph would look like, but it probably exists, and that factor definitely could influence a utilitarian calculation.

If we're talking about torture vs death, if we're using preference utilitarianism, we can say that the point where the torture victim starts begging for death is where that dividing line can be drawn. I don't know where that line is, and it's not an experiment I'm inclined to try anytime soon.