Good question. Got any string?
If doing a "one-shot" session with a group of stakeholders and all you're aiming to produce is a Needs Matrix, it takes about a day. We normally call this a Mini.
Having said that, the quickest we've seen it done is 4 hours.
You can make the session shorter by encouraging the participants to come with some decisions already made. Such as:
business objectives and weightings
"customer" types and weightings
However, we don't recommend it. Much of the value of a Method stakeholder session is in the discussion around precisely these subject areas. Also, not everyone is great at describing an objective and the facilitator has an important role to play: simplifying sometimes complex business-speak and sometimes as an agent provocateur.