You have 12 months to accomplish the assignment - from the moment you have downloaded till the moment you will upload it. How much time you will spend on the assignment within this time limit is up to you.
The time period of 40-80 hours is given just to keep you on track. It is simply considered that if you spend more then 80 hours on your assignment, your architecture will contain too many details that will make it more difficult for a reader to understand it.
No one forbids you to spend more time on you assignment then the suggested time period, just keep in mind that your design is clearly communicated and is not overwhelmed with the details.
The article that you read was reflecting the thoughts of the author. No examiner would fail you if you submit your assignment even on the last day, but it should represent a plausible solution and satisfy all of the requirements listed.