This question might be somewhat subjective, but I hope it doesn't get closed because it is very relevant. Especially for upcoming designers.
I work 8 hours a day. I am employed, but at a cooperative design studio / print shop, so I'm also a co-owner which of course gives me some extra (unpaid) work (and worries).
Sometimes a tight deadline or the opportunity to gain a new client forces me to work 10-12 hours a day and sometimes there is low activity and I can take some time off. But the average is about 8 hours a day.
I must admit that the job does affect my personal life. If I have a really difficult task waiting next day, I can't help thinking about it all evening.
I try not to take the work home with me, but sometimes it can be a relief to do it because it feels better to just keep working instead of sitting doing nothing, worrying about how to get the work done the next day. This is not recommended though! If you only live in "InDesign mode" you forget to look at the world around you and get inspiration from art and life itself.
Besides working on projects for clients I tend to spend 1-4 hours every evening exploring different aspects of graphic design. Looking at other peoples work, trying out ideas I get during the day, learning about the applications and checking out new alternatives, studying art history, geometry, typography, math, color profiles, drawing, photography, printing methods, marketing etc. Answering questions on Stack Exchange. It is endless!
Those extra hours are sadly unpaid and if you think it seems unfair it might not be the right job for you. You have to be a bit nerdy and have a personal drive to keep yourself updated, seek inspiration and have fun doing it. Otherwise the job might seem too hard.
Graphic design is (in my opinion and where I live - Denmark) greatly underpaid, compared to the value it adds to the customers product and it is an ongoing struggle to convince the customer to pay the true price for what they get.