Every placement I have been on has been very clear about the working hours. Usually it's 9-6, with an hour for lunch, to something to that effect. I rarely see a lunch break and certainly not one for an hour. Most of the studios I have been to sustain themselves on a liquid lunch poised in front of their Mac.
At the end of the day, it is always really awkward leaving. Home time comes and goes, and the entire studio continues working; tight deadlines or just a preference to avoid the commuter rush lead to late nights for most designers and account managers. I always find myself leaving a customary 15 minutes later than home-time, even if I haven't got anything left to do, just so that I don't appear eager to leave.
I have heard stories about friends interning, whom have felt obliged to stay up with their studio and work into the early hours of the morning. When I was a student I only did this on maybe two-three occassions, and the time management failure was no fault of my own. I pride myself in being able to do the amount of work in the set amount of time, and so resent that people are seen as more hard working if they stay later. If a studio needed me around until 5am because of some important new business pitch the following morning, I'd question how organised and well prepared the studio actually are. After all, this isn't something they are new to.
At the end of the day, my health and sanity comes before the agency's need to have well mounted presentation boards for a meeting the following morning. As an intern, I have never felt that integral to a project where I have felt that they couldn't possibly do it without me and perhaps it would be nice to feel like that; then I might stay. But as it stands, you'll be lucky to catch me in the studio twenty minutes after home time.