How long do work studies last?

I know that class 1a's work studies (Nighteye arc) only lasted about a month the first time around, and the second lasted maybe three months appaerntly (Endeavor's agency arc), but then I also saw Mirio mention in chapter 126 that he was interning with Nighteye for an entire year at the time.

How long do work studies last typically? Is the difference dependong on their year or varied between who they choose to work study under?