I would expect it to be 20 seconds. That allows you to use it as every second combo to keep it up constantly, or every 3rd combo and have it down for 5 seconds between applications.

Funny, by making Storm's Path this good, they have simply swapped the combos that don't get used, 11% more damage done by the tank (who doesn't need the extra hate and who doesn't really care about damage for IB anymore) or 10% less damage received. I know which one I will be using.

Depending on duration of the debuff though, this will change most people's combo rotations a fair bit.