To answer your question: The 2 week period begins at the time of purchase.
I have been watching the prepcast videos all day and all night. The videos are very good and I am learning many new things in regards to project management. However, the prepcast was purchased innitially by an associate, then I had to go thru the process of changing the prep cast information in my name. When does the 2 week clock start? Does it start at the moment of first purchase thursday 12/16/2010... or at the moment that I downloaded the cast in my name Monday 12/20/2010...??