Compound nouns and sentence structure

Hi there, can anyone tell me what part of speech "20 years" is in the sentence below:

EX: 20 years is a long time.

I am confused since 20 years is acting as the subject, is the '20 years' then considered a noun or a compound noun? How can 20 years act like on thing and now be the focus, can someone please help me, I am just stuck.