To make your Spoiler hidden, what Orangeus did was change the font color to white. It is almost invisible against the background on here.
The absolute minimum should be two weeks, but I think all HP7 posts should have "Spoilers" in the title of the thread, so that people who haven't read the book (and care) know to stay away. If people do read a thread like that and get spoiled, I don't have much sympathy. Also, thread titles should be vague: "Harry Potter 7 Spoilers In Here."
It was Colonel Mustard, in the Conservatory, with the wrench.