It's up to you, how hard do you want the challenge to be? It could be challenging to do everything within one hour. It could be technically challenging to only do the implementation within one hour. Or it could be mentally challenging to get an idea within one hour, maybe without doing any implementation afterward...
I set up the challenge personally to provide me regularly with a burst of creativity, so I'm not very strict on the deadline, as long as I get what I'm looking for.
So, what are you looking for? ;)