The phrase "work many hours" is grammatically correct and can be used in written English.
Example: "I have to work many hours this week to meet my deadline."
But "he didn't work many hours and ended up hanging out at home a lot.
If you are a student or a single mother, you can't work many hours".
Most full-time employees work many hours of unpaid "service overtime".
This means they demand fewer goods and are willing to work many hours, which leaves China as a net exporter.
When I can, I work many hours to make up for how little money I am being paid.
I work many hours after school because I want to be the best teacher I can be.
We've learned that students are working many hours to make ends meet.
Family members of MPs' staff save the taxpayer money by working many hours in unpaid overtime.
He also paid 100 to 150 employees their regular wages, although they worked many hours' overtime with no additional pay.
He works many hours a day and spends many hours a week greeting people in St Peter's Square in the freezing cold".
Like many leaders, Ms. Williams said she worked many hours unpaid — ordering products, delivering them to meetings, counting and banking the money collected, and preparing her remarks for meetings.
