I think that theoretically any number of years short of infinity should be within the realm of plausibility with the caveat that it doesn't overcome the ultimate objective of benevolence by making it impossible to enjoy the benefits of the suffering. That is, the suffering isn't a means to itself, it must lead to greater benefit of pleasure. So any amount of suffering is plausible so long as it raises that benefit and doesn't impair it.
Infinity doesn't work like that, though. There's no "years short of infinity," something is either infinite or it's not. There are different cardinalities of infinity. I don't think it's a reasonable idea that someone could be tortured for 1,000,000,000 years for a "good, unknown reason," let alone 1,000 or anything of the sort. This drives home the point that if special pleading is allowed, then we can get around any deduction or any reasoned position by simply saying "the contradiction isn't REALLY a contradiction in an unknowable way." It's a non-response. One might as well respond "well, maybe it really IS a contradiction in a further unknowable way," or something. This is why special pleading is fallacious.