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.
Those who dance are thought mad by those who hear not the music. This tiny ad plays the bagpipes:
Devious Experiments for a Truly Passive Greenhouse!