The use of which verb is correct?

"3 generations of stupidity is enough." OR "3 generations of stupidity are enough."?

P.S. in the book Grammar in Use (advanced) the author suggests that when the complement is a singular noun phrase (though our complement here is an adverb (=enough)) a singular verb must be used. I believe the exact example was "3 hours is a long time (=the singular noun phrase) to take on the homework." However, I once heard a lawyer use "are" in a very formal and eloquent speech before the judge, which I think negates what the book says.

Solution 1:

The principles relevant in this case are "1" and "22" int he following reference: UWF WRITING LAB.

1 A verb must agree with its subject, not with any additive phrase in the sentence such as a prepositional or verbal phrase. Ignore such phrases.

Example: Your copy (of these rules) is on the desk.

Example: The video will not be available until the dispute (over video rights) is settled.

22 Nouns expressing time, distance, weight, and measurement are singular when they refer to a unit and plural when they refer to separate items.

Example: Fifty yards is a short distance.

Example: Ten years have passed since I finished college.

In your sentence there is evidently the choice of considering "3 generations" as either a unit or as separate items; it's a matter of context. The two examples I constructed below seem to fulfil respectively the two criteria.

• Since my great grand-father started studying the question until today we all failed to understand that, but 3 generations of stupidity is enough, we have got to do something about it.
• My great grand-father didn't understand and thought nothing of it, nor did my grand-father and my father who thought no better in connection with this question, but 3 generations of stupidity are more than enough, I can do something about it and will.