Let's say your object is named propeller and you want to rotate it around the Z axis. Then the expression could be as easy as:

propeller.rz = frame;

If you want to control the speed, you could add a custom attribute to your object (let's say speed) and change the expression to:

propeller.rz = frame * propeller.speed;

This way you can key the speed attribute and control the speed of the rotation.