The first two years were kind of like a nondemanding job for me. I'd say I put in about 40h/week or less, on average (less during non-exam times, more during exam periods). I skipped most of my lectures, studied in the mornings, and went to group sessions and practicals in the afternoon. Was usually done by 5-ish; would go for a run, get dinner, maybe study a little in the evening but goofed off a lot too. I had a boyfriend in another city and used my weekends to hang out with him.
Note that I knew from the start that I was not interested in derm/ent/neurosurg and figured I could afford to slack. My grades were squarely mediocre, I think I was at about the 40th or 50th percentile in my class for grades and ended up around the 66th percentile for Step I.
Third year varies a lot. I have yet to do surgery (which is supposedly the most time-intensive rotation). Some of the rotations were pretty cush (psych and primary care were 40 h or less, and we had some random subspecialty weeks that were <30 h/week). Medicine averaged maybe about 60 h/week depending on call. Ob-gyn was maybe 60-75 h/week depending on the call (10-12 h/day on non-call weekdays, if you had call that would bump your total hours up). There was some studying in the off time but I got most of mine done during down-time on the wards. I ended up with mostly passes and some honors in the rotations I especially liked.
(As a comparison to this, in grad school I logged about 70 h/week without overnight call (typically 11-14 h/day on weekdays plus a variable 4-16 h per weekend).)
People who want to stand out in med school have to work harder than I did. But if you are not gunning for a super-competitive residency there is no reason you can't have a life during med school and still learn everything you need to know to be a good doc. Intern year may be a different story.