Notice how you've got a great basis for
estimating now? You've got a good idea of the number of processes to
investigate and a feel for their complexity.
You can start booking interviews with the
various business reps to cover off the decomposition.
You should have a rough feel as to how long
each process investigation will take.
If users can't devote the time when you need
it, it is relatively easy to see whether the various milestones in the
project plan are jeopardised. If so, have speaks with the project
manager.
..an early warning system :)