As the other commenter pointed out, you don't control how many hours a candidate spends on the assignment. All you see is the end result, and the guy who spends

As the other commenter pointed out, you don't control how many hours a candidate spends on the assignment. All you see is the end result, and the guy who spends 8 hours will generally end up with a more impressive result than the candidate who spends 1 hour, and this reflects time spent far more than candidate quality. If you give an "open ended" assignment, you are encouraging candidates to spend a ton of time and then lie and claim they spent very little time, because that looks impressive. Do you really think "the 1 person who made a proper package, a short .rst install & use documentation, and included crazy unicode test cases" spent only an hour?