It lasts 15 seconds. 15 seconds * .25% mana / second = 3.75% of your mana over 15 seconds.
It grants the exact same buff as the Ret pally's.
We can cast our Mind Blast every ~6 seconds with talents, which is similar to how often a pally can judge. Hence, there's really no difference between our way of granting replenishment and a Ret pally's.
Basically, it's not going down. As long as you're doing your job, Replenishment is up.
This is the same for all the other classes that grant the buff (Frost Mage, Destruction Warlock and Survival Hunter).
Edited, May 10th 2009 11:13pm by IDrownFish