About 5hrs over the weekend.
I was the lead dev since I had experience building web apps (but not facebook). I setup the AWS environment, Grails app, and worked on the core domain model (which switched multiple times) while the other dev spent most of his time figuring out how to login to facebook and pull friend lists and post status updates and such. It was just the two of us while the rest of the team worked on company strategy, setting up twitter, facebook, wordpress etc. A couple graphics guys to provide a few images here and there (many which didn't make it in yet) and they even made a 90sec video spot in case we won.
I think the weekend was worth the experience, but I'm not super anxious to go do another one right away. But the community was great and the atmosphere was the best part I think.