Two days, up to ~14 parallel sessions, and somehow, it works. Talks are much less formal (and also less tedious) than a typical conference, and people in the audience tend to be interested enough to speak up. Many of the spontaneous sessions had around 20 people in the audience, but some (James Randi's talk) had many more.
Hackers, another conference I go to, does a hybrid approach with 3 parallel talks and a lot of spontaneous BoFs (birds of a feather sessions).
One of the key ingredients to an unconference - a sufficient number of small rooms so that you can have lots of 20-30 person talks, combined with good open space for people to hang out and meet. Large convention centers often lack the latter - San Francisco's Moscone Center's meeting halls are too large, and the food areas (where people congregate) are too impersonal and open.
Small hotels, or large company spaces on weekends (thanks, Google) seem to have the right mix of spaces.