What's the maximum amount of the worst suffering a classical utilitarian should accept for 50 years of the best well-being?
I'm not sure there is an objective answer to your question because (to my knowledge) we don't have good definitions of suffering and well-being and no objective way to compare their intensity. It also depends on whether by "worst suffering" you mean the worst suffering current humans can experience, or the worst theoretically possible suffering (and the best theoretically possible well-being). Some speculations related to the latter can be read here. You can probably find more such speculations by going through these search results.
If you are asking about current humans, it may be informative to ask people who have experienced both extreme suffering and extreme happiness for their opinion. However, in my experience different people answer such questions very differently. Also, I imagine that even the same person may answer the question differently depending on whether you are asking them while they are experiencing intense suffering or after it has already happened. And it's unclear what to do with that.
I'm broadly with Saulius on this one: it depends entirely on what you mean by "the best well-being", what kinds of agents are experiencing it, how many of them there are, whether all the numbers involved are even finite, etc.
But for a thoroughgoing classical utilitarian (who believes you can in principle measure happiness and suffering objectively), there is an answer: , where is the total counterfactual happiness gained across all agents during those 50 years, and is an infinitesimal number.
(This is assuming there's no utility cost to taking/implementing the deal other than what's "on the label", as it were. If there are transaction costs then you need to take those into account as well.)