Microsoft's IOSTRESS test - how do you control the duration

Yes, I know there’s a duration box in the GUI where you fill in the number of hours & minutes you want it to run.
It’s just I can’t seem to get that to work.
I would like to be able to automate a run - kick it off for so many hours and have it automatically terminate rather than have to manually stop it and then wait ages for all the cleanup and unwinding.
Using various versions depending on the platform but even the vsn. from September’s Plugfest is the same.

Any useful bits of info welcome…
regards Maria H