That's usually the understanding everyone has by default, unless you're doing some specific job that requires greater availability (and are compensated for it), or... you're eroding this understanding yourself by doing something unwise, like replying to company e-mails or IMs after hours.
One rule I learned to stick to: never install apps for company e-mail and IM chats on your personal smartphone. Just don't. Otherwise, you never truly "leave the office for the day".
We've had to explicitly tell just-out-of-college new hires to stop at the end of the workday. The reasons are different for each but a lot of them have gone home and kept working without being asked or expected to.
Plenty of places just assume you can do on-call or it's required, including high-tech companies. I'm not compensated outside of normal salary/equity for on-call hours. If I don't get paged, great. But if it's a heavy week I don't get anything extra.
I'd love to join a union to be able to reasonably bargain against this.
...that's how I've been forever, aside from those few jobs that required more availability. In those cases, it was part of my job description, so agreed upon by all parties before I took the job.
