OP, as others have said, this really depends on the person. I was a little freaked out going into med school because I'd heard all of the "study all the time" hype, and the only real studying I'd done in undergrad was a few hours of cramming the night before or morning of a test. The thing is, in undergrad you know you can pass without studying, so there's not much motivation. That totally changes in med school. If you don't study, you fail, so you suddenly have a VERY good motivator, and studying becomes a lot easier.
Also, it's all about studying smarter, not harder. I studied a ton (for me) in 1st year - probably averaged about 8-10 hours a day of lecture + studying - but it wasn't very efficient. Second year I got smarter about how I studied (no studying while IMing, watching TV, etc; figuring out which resources were really high yield; making it more active, ie read some and then ask myself about what I'd just read versus just reading) and got the same grades while only spending about 5-6 hours a day on lecture + studying. Another thing that totally helped in second year was watching lectures webcast rather than live. I can never sit through and pay attention to lectures, so it was great to be able to pause whenever I needed a break (I retained a lot more that way too), put slow talkers on double speed, and altogether skip lectures where the professor was just reading his notes. If your school offers this, I highly recommend it.
You'll find something that works for you, so don't get scared off by stories of 24/7 studying. Med students like to exaggerate this anyways since we love thinking of ourselves as martyrs. 😛