Isn't that one person submit one entry ?
As far as I know, that's allowed, I just don't remember the specific thread in which it has been mentioned. Plus, I am certain it is also the reason why multiple categories exist in the first place.I don't think someone can create a good art for 1-3 days only because you need to prepare anything about the concept and idea which can take a lot time, not to mention everybody have daily routines and busy.
Some people can! Others can also do an extremely aesthetic piece less than 24 hours 