I don't think it has to do at all with the leaders time (yes the leader must put some time in) but it all has to do with the level of competence and maturity of the leader.
The successful groups tend to have leaders who are adults and living on their own. These people have jobs and are able to work in a wide variety of areas. There is just a certain stability brought to a group by 20 somethings on up that the primary members in their pre and early teens can't bring. What is that you might be thinking? Identity. They know who they are, what direction their lives are going (for the most part). Most groups led by 20 somethings or older actually succeed because they can make effective use of the time they have.
the other thing is size of the group. i don't believe in capping a groups size, but realisticly the smaller tighter knit groups last longer. There needs to be some organizational skills. The group must have an outline either for expansion. (if anyone wants to know knight watches, just ask, i'll tell you). the ability to make a group small and seem personalized to each of your members while retaining the groups intent. If you as the leader or higher level members of the group appear to not have time for your members, then you aren't doing them any good.
the biggest thing is once you make a set of rules, stick to them. ammend them if they'll make life in your group smoother. As long as people are nice and active, return the courtesy. If some one never gets active, start emailing, messaging, etc to them to find out why. Eventually you don't have to be nice. Kick them out.
I include this to every new member we have, "welcome to your new online home. we hope that above all else you will have friends, make new friends, and enjoy the time you put into this group. it's not whether you win or lose this game, (although we hope you do win) it's that you found a group of people who enjoy playing this game with you. If we do that for you, then we have succeeded."
*steps down from pulpit*
aka Bad Monkey
Knight Watch Shipping Corporation