I would go even further with a 4 second duration and a 10 second cooldown (starting after said duration ends). 8-6 seconds can be a large chunk of a teamfight, which is a long time to be empowered by a self-inflicted ability.
I, personally, also want an E ability that can be used on a consistent basis. An 8 second duration and a 12 second cooldown is a combined 20 seconds between activations along with an uptime pushing on overpowered.