Entirely depends on what they're doing. I mean honestly, I don't expect them back home...they always come home at a reasonable time though.
They might somewhere with friends and they always catch the last train home which is at 10.30pm but if they were at a party, I'd expect them to get an Uber so they could come home later.
Usually though, they will arrange a sleepover at a friend's who lives closer to where they're going.
What I am trying to say is that at 17 it's largely up to them as long as they let me know their plans and where they're going so I don't worry.
Mine always texts "On train now" or whatever. Or "Arrived at Bob's house, we're all going to stay here"
And that's that...they're nearly 18 after all.