An article in The Boston Globe tells us that "the night's arrangements took a month or two to prepare."
I haven't seen it, but how long can it take to put a few pumpkins around and throw up some toilet paper. I can think of bunches of people who would love to TP the White House who would've done it and they would have been in and out lickety split.
With all the power and resources of the United States Government, a union grounds crew, and numerous staff members, it still took two months. Maybe there was a standing comittee formed to asses the validity of the decorations or maybe a vote was held by The Obamas and their advisors to determine if the hanging spider should be a black widow or a daddy longlegs.
If this is how long it really took to decorate the White House, imagine how efficient the government would be with supplying HELLthcare to all.