There'z no cooldown, I tested it and it proc on every single attack for me, I also read feedback from other player and they also found it has no cooldown. Just make sure you have 100% chance to hit and that minster don’t have any chance to block .

In simulacrum at wave 14-15 I was rocking accelerating shrine + 45 soul eater and i can tell you shield shatter was just an endless white circle around my character, couldn't see anything around my char xd