Depends on what type of person you are. Don't go into medical school thinking you will never have to study- if you do this you will probably end up being depressed. Don't go into medical school thinking that you have to learn every single thing and do everything perfect. If you do, you will probably end up depressed.
Basically, I went into thinking I would be studying 24/7 & no breaks just to pass. I told myself I enjoy studying (ha ha) and that no matter what I would grit my teeth for 4-8 yrs and bear it. I was pleasantly surprised that it is not like this... at least I don't let it be like this. Get a feel for how things are after the first exam and find out how hard you have to work to do how well that you want to do. You will eventually become a more efficient studier and will only put fourth the amount of effort required to reach your goal. If you study 4hrs a day, everyday (Except fri night and sunday), and increase this (read: double this) amount before exams- you will probably do just fine. Make sure to take time everyday to do something that makes you happy!
Moral of the story: Expect the worst, and you won't be let down, and you might actually be pleasantly surprised!