It seems that the cases that go the distance (2-3 years or more) tend to be single cases of company A vs Company B. That may not be accurate but it is just my observation after following peripherally some high profile cases.
I think that when you are in EDIG's position of "OVER" 174 identified infringers there may be a tendancy to get to settlement earlier. PTSC seems to score a settlement per month now that they are through their preliminary stages.
Just a feeling and I am sure there is no hard and fast rule, but if 18 months out EDIG is settling an infinger per month we should be in the promise land.