Very long .... I have just checked the ones I have been involved in and they were four to six months. Island of Sorrow was over a year though.
I think a lot depends on the structure of the story and the context.
Darker Days was a relatively simple plot and because it didn't relate to the "known" history of Middle Earth the writers were not too restricted by "known historical facts". However
Red flows the Sirannon came in at under six months despite the historical context but the plot was still simpler and had a lot of experienced players.
This kind of complex set up where there are lots of separate groups make it particularly hard to gain momentum....
I can only conclude that minimally historical journey style RPGs with only one or two groups are the easiest to keep going.. at least that is the way I shall go if I ever dare take the plunge...
