The Resist Songs are suppose to be instant cast, and the pulse should last 5 seconds.
Unlike all the other Songs, the Resist Songs don't have a re-use delay timer.
Resists Songs should have a 8 seconds re-use timer/cooldown/recast.
First hit on a Google search from IGN boards:
Said Google search:
Sources showing the newer combined Resist Song having a Recast timer (1.78+):