Will the expectation be the same in all types of firms?
Asking particularly in the context of US firms where lawyers tend to work longer hours. Will VS students who leave at 1730 be seen as unsuitable for their norms and culture?
Yes. The expectation is the same across firms.
There is no need to stay late unless there is work to do. If you have concluded your work for the day, ask your supervisor if there is anything else you can help with for that day and if they say no, you leave.
Sitting around doing nothing and distracting other people will be a much bigger risk than going home.
Obviously if your supervisor has given you work to do and you need to do it to meet the deadlines set, then stay later. At firms where the culture is known to be late hours, then you might find your supervisor has given you work that means you have to stay later.
This is going to sound brutal, but many supervisors will want you to leave for the day so they can focus on their work. Looking after a VSer is hard work and can often be a massive distraction - getting a few hours to themselves at the end of the day probably helps them deliver the work they need to do (and that you can’t help them with). If your supervisor tells you there is nothing to help them with for the rest of the day and it’s gone beyond your contracted hours, go home!
If you are still worried use the “after your contracted hours” period to catch up with other VSers, organise end of day coffee catch up with employees in parts of the business you want to find more about, or go to a communal area and do things like write up a day’s journal of a summary of what you learnt from the day/do some reading.