Obviously this will depend on the scene that you are wanting to photograph. If this is a street scene, what objects are within the scene that will cause an individual to become distracted, stop, and observe. If there is nothing then the time would obviously be less then if one were within an art gallery or a museum for instance. Once you have decided what the scene is and what the propensity of peoply stopping within the scene, I would then meter a white cloth in the lighting conditions that I wanted to photograph in. I would place this value on below a zone I placement and I would then make a selection of film and ND filtration to give me the time that I had determined. Don't forget reciprocity considerations in your calculations since they will alter the actual exposure calculation by several stops at the very least. If my memory serves me correct, I seem to remember those examples of this technique that I have encountered used slow films and ND3 filtration.