First, engineers can work varying amounts during the week or even a day. But on average, statistically, engineers work more than 40 hours a week. That's AVERAGE. Again, as I started out saying, that means a lot of engineers work less and a lot work more. Now, if you're in a support role -- that is, supporting the activities of others, either co-workers or customers or other folks, you'll likely work during their average 9-5 shift. If, like me, you like to get involved in some challenging projects, possibly managing teams in other countries, you'll work when you think you need to get something done. And if you're lucky enough to get on really cool projects like I generally did, you'll be thinking about solving problems and getting new ideas all the time. And you'll love it. At one point, I was lead engineer on a large project that took up 140 hours a week. You read that right. 140 hours or even more. That left only a few hours in the week to sleep. And I loved it. But that's very, very rare, and very indicative of the fact that people involved in science or any other projects of passion get fanatical about the rush of solving problems and achieving. Getting your first patent or being asked to present your first discovery or creation is a great rush and encouragement.
As you move up the ladder, you'll have more control of your hours, but beware -- you get hooked on the "solving problems" thing, and you'll often feel compelled to work much longer. But that is voluntary.
So the classic answer is "about 40 hours a week", but compare that to a dedicated musician who only thinks about music "about 40 hours a week". Probably not going to happen. These folks will likely be thinking, eating, sleeping, listening and playing music most of every day and often much of their dreams. If your job was pumping septic tanks, you'd be working pretty hard and feeling a sense of accomplishment when you fix someone's plumbing problem. But you're probably less likely to be spending a lot of your free time doing that. Some jobs are more consuming. And hey -- that's a good thing that people can have jobs that they'd do for free anyway.