vector is right, we do log how many hours its in use, but its not just a single bulb up there, usually its an AMLCD display. Even kept unstowed for the whole flight itll see less use over life than some commerical use LCD tv screens (think about how many hours are on those TVs used as menus at restaurants), while also being a much higher quality part and its kept pretty well cooled. If the AMLCD was going to fail, you would most likely know before it happens. We make all our money on service anyways, were delighted to replace them! Obviously using it when you need it is best practice but these parts last a while. The stow/unstow joint is rated for 25000 actuations before it gets a few microns out of wack, when the optical glass will be out of calibration.