Bashiok mentions in this thread that Fury decay is not immediate, and I've observed that to be the case myself. How long does it take for Fury to start to decay? Are there other factors to consider, or does it always start after the same amount of time? Does gaining Fury under always reset the timer?
I just tested this, and in 5 times it was approx. 6 seconds after I ceased attacking. So I attacked and then ran away and watched my Fury. After 6 seconds the decay began each time. Gaining Fury does in fact reset the timer. There are a few skills such as the passive Unforgiving that totally eliminates Fury decay. There are also other items that can alter the rate at which you gain Fury, but I do not think there are any that alter its decay like Unforgiving does.
Also, when using a shout as a Fury generating skill if you are not in combat the decay will begin instantly.