An "optimal" amount of time? So we expect "good enough" solutions to be materialized in a matter of an hour?
If my PM ever walked up to me and said, "Hey, I have this project, and I need you to have it back to me in an hour," I would laugh, then escort them over to their boss, who would also laugh at them before they walked back to their desk in shame.
It's not testing anything but whether or not you're good at studying for exams.