From the CADisplayLink class reference:
The target can read the display link’s timestamp property to retrieve
the time that the previous frame was displayed. For example, an
application that displays movies might use the timestamp to calculate
which video frame will be displayed next. An application that performs
its own animations might use the timestamp to determine where and how
displayed objects appear in the upcoming frame. The duration property
provides the amount of time between frames. You can use this value in
your application to calculate the frame rate of the display, the
approximate time that the next frame will be displayed, and to adjust
the drawing behavior so that the next frame is prepared in time to be
displayed.
Meaning the frame rate is determined by the system, and you should base your animation off that. There is no way to set the frame rate, rather it will be determined by a combination of factors (e.g. how processor intensive active actives are, how graphic intensive the UI is, etc.).