The wheels come with a microchip that talks to the machine so I don't think you can go any longer than they allow. I haven't used a full ring yet, but I believe the machine won't use a spent wheel.
Speaking with the owner of the company about the quality of sharpens near the end of the wheels life, he said they set the expiration before any loss of quality. So the first sharpen should feel the same as the 40th one.