word choice - Is it "5–6 weeks are a lot of time" or "5–6 weeks is a lot of time"?

I was just copyediting somebody's answer on another SE site and my native English speaker Sprachgefühl told me I had to correct the grammar of one sentence:

... 5–6 weeks are a lot of time ...

by changing the are to is. But as I was doing so I started wondering why is it that in this case it seems that I have to make the verb disagree with the plural subject?

So is my feeling for English going bad or if I did the right thing, how could I explain this to somebody who's learning English for instance?

Answer

Use is because you're talking about a single period of time with a range-based duration.

There are extensive discussions of the subtleties of Collective Nouns and Mass Nouns on Wikipedia that explain from a technical perspective why some seemingly plural things are treated as singular grammatically.