Can I replace "80-hour weeks" to "80 hours a week"?
You can replace "80-hour weeks" in this sentence with "80 hours a week", however, it would slightly change the nuance of the sentence. This is because "80-hour" is an adjective used to describe "weeks" in "80-hour weeks". If you changed the sentence to "he regularly puts in 80 hours a week", this would take away the adjective. While the second sentence has the same meaning, it is different in nuance because without the adjective "80-hour", it makes the sentence sound slightly less strong.