edit: Tested with @Rom, appears to be about 500-600ms.
I'll log in so you can test it.
So far, the primary use for it seems to be delaying restoration salve on initial breaks, and it's really great to that end (although, unfortunately, you'll typically want to save ferocity for proneing, unless you're confident you can get an isaz proc or paralysis for impale).
It's highly valuable for rift locks, for the same purpose.
In almost all other situations, I find it best to use it for sensi (if they don't already have it, of course). It strips deafness first (as is usually the case with sensi), but it really helps build up a kelp stack, and helps considerably for building up a kelp stack. Keeping deafness down with sensi might also be useful for battlecry, for people who find a use for it.
Answers