should I use singular or plural verb

Question

It's a sentence from my dictionary:

Years of smoking have taken their toll on his health.

I'm wondering why it uses "have" instead of "has". I know "years" is in the plural, but "years of smoking" is just a single thing at large. The point is on "smoke", not "years". I can just simplify the subject as "smoking" if I want. But it would be wrong if I simplify the subject as "years" and omit "smoking" because time is not the reason, smoking is. So in this case, I feel the following sentence is right:

Years of smoking has taken their toll on his health.

But the right verb should be in the plural, why? How to explain this?

Answer 1

Mechanically, years is the plural subject, and requires a verb in plural concord:

Years ... have taken ...

However, you may also with perfect propriety treat Years of smoking as a ‘single thing at large’, and use the singular verb in concord:

Years of smoking has taken ...

But if you do so, you must do so consistently, throughout the sentence; a singular subject cannot act as referent for plural pronoun^†:

Years of smoking has taken its toll on his health.

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^† except in the special case of the degenderative they.

Answer 2

Actually it is a very abstract idea. Grammatically both of them are correct. But each case gives slightly different feeling to the reader.
If you are generally talking about a large amount of time spent smoking as years of smoking, and you want to say something about that time then has makes more sense. But if you are talking about specifically years which were passed smoking then have makes more sense.