I'm listing a few different methods that you can use to extract requirements from clients. In my previous post I talked about understanding the situation you are facing: the nature of the project, the urgency and the availability of knowledgeable resources. These factors will determine which methods are more suitable for your situation and what combination you will employ. By using your experience and factoring in the magnitude of your project, you will be able to estimate how many sessions you will need for each method.
My next post will be concerned with setting expectations for your clients as to the production of requirement deliverables.