8 hours of work a day is a full-time job. 

But most people work more, a lot more.  Let's say 12 hours.  

8 x 5 days a week = 40 hours.  

12 x 5 days a week = 60 hours.  

12 x 7 days a week = 84 hours.  

So if someone needs to complete a project in 100 hours, it would take at least 2 weeks.  

How many working hours can one accomplish in a month?  12 x 30 = 360 hours.