I too had many, many accounts on a development clone. Some of which never saw any use and where just there to give other test users things to do. There was a neato v3 plugin of awesome admin tools that had a bulk user creator thingie.
While password expiration is certainly something that's possible I don't believe this functionality exists within phpfox natively.
Unless you've jumped through some hoops to make this happen, it's not happening. However, I will admit I don't know the v4 code at all. I never saw anything in v3 for password expiry. There was a pretty neato add-on that had a password strength meter thing, to help encourage people to pick strong passwords. Still it was not enforced if I recall correctly and there was no expiry or aging hooks either. There's no way that I saw to hook phpfox into say, the linux PAM framework for user accounts. PAM has native password aging and expiry however ...