Ever since I have been using gmail for the last couple of months I have become much more aware of how long, and short, some of my email conversation threads are. It is interesting to see thread counts grow rapidly, and then hit a barrier around 6-7. Very few threads over 7 messages.
I imagine this is the function of the number of correspondents. With 2 participants, 3 iterations (5 or 6 messages) probably get the point accross. With a higher number of participants the number increases, but slowly, since obviously you do not get everyone to contribute 2 or 3 emails each to the discussion. The record of off-the-cuff discussion between 3 people in the last 2 months is 12.
I should probably not spend too much time tabulating these results.