I don't understand your question.
Do you mean you want to rotate the Z axis of your layer continuously? Obviously, the easiest way is to just set a keyframe at the start and end of your layer. Alternately, you can use Loop expressions to loop keyframe activity.
Please post a more specific explanation of what you are trying to achieve.
For a linear variation you can use a simple expression, like
initAngle = 0;
rotateSpeed = 60; // 60 degrees/second
initAngle + (time-inPoint)*rotateSpeed;
(For that, alt+click the stopwatch of the angle property and enter the above text in the expression field).
For non linear variations it is way easier to put 2+ keyframes and adjust the temporal Bézier in the graph editor.