Change the bone's rotation driver to Euler.
In a Choreography, Action or Pose window:
- Select the bone.
- Create a channel for it by rotating it an arbitrary amount (probably around the Z axis).
- Right-click on "Rotate", which is under "Transform" in the Properties palette.
- Choose "Convert Driver To > Euler"
- Go to the frame where you want to bone to stop rotating.
- On the end frame, enter the total number of degrees you want the bone to rotate.