I've been hearing the drop rate for the mirror is approximately 3%, rounded up to 5% just to give a little bit of wiggle room.
Consider this: If you are not running Leviathan X with a static, you are going to be rolling against 7 other people if the mirror drops. So, your odds of getting a mirror are approximately 0.625% from a single run.
With a 0.625% chance, it will take on average 160 clears for a given person to get their mirror. Approximately 1 out of every 25 people will require over 500 clears to get their mirror.
That means, for every 3 groups that run leviathan, there is one person will take over 500 runs to get theirs.
Now, lets look at that in terms of time. Lets say each kill takes 10 minutes including loading buffing, and reset. On average, it will take 26.6 hours with a 100% kill rate, with the unlucky 5% (478 kills) taking over 79.6 hours.
Obviously, these numbers get worse if the drop rate is indeed lower, like I have been hearing - at 3% taking on average 267 kills, with the unlucky 5% taking over 796 kills.
Am I the only one that finds those numbers absurd?