While this seems like a good idea on paper, you know that people are not going to stop after 2 hours unless you force them to (i.e. by doing the assignment on-site).
>get it done in 30 minutes, and intermediate 2 hours and a junior probably not finish.
What kind of exercise could a junior programmer not finish? Give up or produce miserable code sure, but not being able to finish? Do you require some really strange domain-specific knowledge?
PS: The website in your profile doesn't seem to be working.