The purpose of this spec was to test a specification, i.e. in this specific case to check that running time is less than a limit and do not to check what is a running time.
If we see that we are above the limit there is no taste to continue to test the specification.
The situation was that a limit was 5 minutes and a running time is at least 15 minutes, than user is not productive 10 minutes, while he is waiting for the result (practically it rather difficult to switch from one task (checking specification) to another while waiting for the result). 
Then "stop when time ends", that is missing with 'benchmark' and 'timeout' is important.
We can consider their usage in other cases.