I opened and logged into 2018 with my password, then I opened and logged into 2017 with my password, then opened and logged into 2016 with my password and was prompted to update my password.

Does it count how many times you log in and makes you update the password after a certain number of log ins?  I thought it was every certain number of days, but that doesn't appear to be how it works or I would have been prompted to update the password when I logged into 2018 first.