Choose: Six years (seems, seem) too much time to build a house [duplicate]

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?

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.

You use the singular because it's a quantity of time. From this website

Quantities or measurements of time, money, distance, weight usually take singular verbs.

It's not just restricted to time, money, distance and weight;

Fifty milliamps is enough to kill a man.
Three G is enough to make a pilot black out.