I have came across such sentence:
By now, Queen Elizabeth II has been on the throne for an astounding 67 years.
Why do we use "an" here, even though 67 years is plural?
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This is a special case of simple agreement override that happens when a measure phrase is interpreted as a single measure (of time, money, or distance). So in this case, "an astounding 67 years" is understood as a single, continuous time period.
Huddleston & Pullum (2002) discuss this under § 18.3 Further Override and Alternations:
18.3 Further Override and Alternations
(a) Measure Phrases
We have already noted that plural measurement nominals can be respecified as singular for the purposes of agreement and selection within the NP. This carries over to subject-verb agreement, whether or not there is any marker of singular number within the NP:
 i That ten days we spent in Florida was fantastic.
ii Twenty dollars seems a ridiculous amount to pay to go to the movies.
iii Five miles is rather more than I want to walk this afternoon.
iv Three eggs is plenty.
This is the opposite of the collective override: here an NP that is formally plural is conceptualised as referring to a single measure (of time, money, distance, or whatever) and accordingly takes a singular verb. The measure override is characteristically found with be or other complex-intransitive verbs (such as seem in [ii]). In [ii], where the predicative complement is a singular NP, the override is obligatory (*Twenty dollars seem a ridiculous amount to pay); in [iiii-v] it is optional but quite strongly preferred.