they are supposed to last the life of the car but having said that my nearsideone began to look dimmer than the o/s, so I replaced it. From citroen they are about £90 a time but if you look on the net you can get a pair for that sort of money. It is better to replace a a pair because the temperature setting of them determines the colour of the light. You can be stopped or fail an MOT for lights not being the same colour. Also they are a so and so to change and due to the very high voltage on them make sure they haven't been on for 10 mins before changing. They are not unlike ordinary bulbs to change but asre very very tight.