In the expression editor:

pCube1.rotateY = 2 * frame;

Replace "pCube1" with your object name.

Replace "rotateY" with whatever axis you want to use.

Replace the number to increase or decrease speed.

pCube1.rotateY = 2 * frame * time;

This one will make it continually speed up over time, again, replacing things to correspond with what you need.