In my experience, a two-hour meeting with the client and subject matter expert(s) can create the heart of the map. You should be able to determine a goal, identify the major actions needed to reach that goal, and begin to determine why people aren't performing those actions.
After the meeting, you'll continue to work with the SME to analyze what people need to do and what solutions can help them do it. How long this takes depends on the complexity of the problem and any institutional barriers to solving it.
