The signs I've seen photographed in Google Maps (I'm several timezones away and am not about to just go there and check) do not say how long you can expect to take, so maybe nobody felt the need to upload a snapshot of one of those signs. Pretty much every such sign in Austria has that information, I find their estimates a bit pessimistic but that is not a bad thing.
18 miles is just under 29km. I'd feel safe in assuming I could do 20km on a well signposted trail with not too much vertical, although I'd need to do some hikes locally in order to build my endurance up in order to make sure beforehand, and I'd want a "get out of here" option if something went wrong. A 17-year-old has the potential to go further than me at my age, but they do not appear to have prepared this at all. I wonder where they were from, locals should have known better so maybe Vancouver.
I'm more a cyclist than a hiker and have come to the conclusion that Google Maps estimates are mostly for cyclists who are stronger / fitter than me (or have e-bikes) unless there's a following wind. Their estimates do make allowances for hills - take one of their estimates and swap the start and end points, there can be a massive difference if their elevations vary by much.