Edit : I've added a little more detail to the main question here.
There are many techniques and practices at our disposal to gain an understanding of the target audiences motivations and needs when using a website or application. These can of course be carried out face to face, remotely or even undercover but all need to be done.
In your experience how long does it take for you to
- Get a reasonable amount of information to base personas and scenarios upon?
Edit : how many interviews from the same demographic or target group would be recommended to base one persona or scenario upon?
Document the findings into deliverables suitable for client use?
How receptive are clients if the effort required is more than they expect?