It depends if you want to give people time to do things like pretty pictures of monuments as inserts around the border of the map. Two months would probably generate greater variety of entrants since it would allow time for the more extravagant styles.

I think a lot of people can do a top view city plan in a week - more so these days with useful aids like Watabou.itch.io to get something going really fast. Its the ISO styles and fine art illustrations that take longer - especially if you don't use Blender or Sketchup at all.