Cutless bearings wear out, and need to be replaced, when you can grasp the prop and move it laterally or up and down a 'noticeable' amount. While this movement is mostly a subjective distance, if you can move the prop and shaft in any direction other in the clockwise or counterclockwise rotation it normally would go in when turning, it's probably time to replace the cutless. Once the bearing begins to wear if you let it go too long prior to replacing it, you will begin to get some vibration from the shaft that is telling you that it is no longer correctly aligned with the transmission coupling... Don't wait too long.