Aside from finding the news that announce the opening, your best bet is ninja exam hero thingy.

When someone passes that, it records the exact day # since server open that they passed it. So if anyone passed anything recently, you'd get a fairly accurate result.

Thou since there is a limit of 20, if there are more than that passing 180 that would be useless.

Sadly, the old metric of survival is messed up due to the 4.0 transition. Before that, you can just take top score, divide by 12 and add 1-3 and get it... But now.. It's not linear....