If the objectives of the list are clear, the discussion well-moderated, and the participants committed, a list can continue indefinitely. The best discussions involve the right people. Recruitment of, and support for, the participation of individuals from developing countries requires a serious and sustained commitment of resources, particularly human resources. This leads to a related point. These dialogues are enabled by the Internet, but they need not be limited to those with Internet access.
Nothing exceeds like success.
Sometimes a list discussion can be too successful, in terms of numbers. If you plan to build a valuable and focused on-line dialogue, you need to devote adequate human resources to it. It cannot be viewed as something to be done in your spare time. Effective moderation of a focused, intensive, productive list discussion can be more than a full-time job. This is all the more reason why lists work best when they are focused and time-bound.