How fast can a company be created?
A weekend appears to the be answer. An interesting approach is being taken by Startup Weekend. Invite people (over 50 showed up), give them an idea (poll your friends via phone), put them in a space for a weekend and see what happens.
For an interesting read on what comes out and how it is progressing check out the first idea --http://www.vosnap.com/. While not fully formed yet, it is interesting to watch.