There is no time limit that I know of. Once you download the assignment you can take as long as you like. If you are reasonibly quick in picking up new skills, you should be able to do the assignment in about 120 hours of work. Your mileage may vary.
My favorite is a chocolate cupcake with white frosting and tiny ad sprinkles.
Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop