I have the normal polearm which, on proc, provides +107 agility buff once a sec to a stack count of ten and then persists for ten more seconds. The LFR version provides a stacking +95 agility. The heroic provides a stacking +120 agility. With some modeling and a 45 sec ICD, I expected the normal to provide approximately an average of 365 agility in the best case scenario when averaged over the course of a whole five minute fight.
For the record, I expected the LFR version to provide 324 and the heroic version to provide an average of 410 agility.
That was very optimistic and simplistic modeling. I was guessing I could get it to proc within the first second when it really only as a 15% chance to proc per shot. Put another way, I was expecting to have some level of the buff (from one stack to ten stacks) for 44.67% of a five minute fight.
What I see in practice is quite different. Below is a heroic Ultraxion and normal Gunship from last night. I expected to see much much better buff numbers and uptime from Ultraxion. I see only 27.9% and 31.7% overall uptime of any stack count. In the below images, the green zones are when the buff is active (at increasing stack counts):
Given these observations I'm thinking my optimistic theory-crafting is supplanted by the observed data. My average uptime is really only about 30% so my average agility for a fight is about 68% of my original calculations. So the average agility gain from the proc would land around these: