ditto .. only about six to eight months with it now, but like ralphie I keep mine suspended off the bottom of the tub, away from the paste.
Not sure if that helps, but I also clean it in a vinegar bath every now and again .. I simply put the pump into a container with vinegar and water, put the outlet tube into the same container and then lift the sensor for a few minutes at a time. This just circulates the solution through the pump for a while .. my goal is to loosen any build up inside the pump and tubing, i think it works.
Having a spare pump on hand is a good idea though, I'll have to do that.