In Bones, after Zack's face-heel turn into the Gormogon's Apprentice ended with his incarceration (commitment?), the assistant internship (fellowship) position was filled with various people, all grad students of Dr. Brennan. These Squint Interns (or Squinterns as Booth once called them), Some left, some came back, but since Dr. Brennan couldn't choose, they eventually fell into a sort of rotation. As we see on screen, each episode had one or the other, almost never twice in a row.
How long were these rotations or shifts? This may essentially be how ever long in-universe between episodes, but even that isn't easy to discern from just watching the show as dates are rarely given (I'm marathoning it, at season 5).
And what exactly was the nature of the rotation? Did they alternate working directly with Dr. Brennan and other duties at the Jeffersonian (Like cataloging bones in "Limbo"), or was their duty only limited to working with Dr. Brennan?