I bought an aftermarket kit that is all Phillips parts. rebased "bulb". Bought them in 2004. They were in 2 of my cars since then, then they sat for about 2 yrs in my tool box. and now are in use in a friends Miata. Still as perfect as the day I bought them...(for WAY more than they cost now).
I've had HID headlights on all of my recent cars (only the Lotus was not OEM HID), and I've not had one failure. My 911 TT is almost 13 years old, and is still on the original bulbs (granted I don't use it that much).
D2S and D2R HID systems are measured by number of "hours" used as it lifespan, the standard adapted time is between 10,000 ~ 15,000 hours. So the less you use, the longer them seem to last overtime vs. someone who is constantly using it.
HID lights also change color hue over time, IE a 4300K OEM will be more bias to white in its first few years, then it changes to a more yellowish hue, and then finally, it will reach pink before it dies. A 6000K will be even more white, but has a higher concentrate of blue, before going through the yellowish ~ pink hue state.
When changing HID bulbs, always change in a pair, as the older one will have a different color to the newer one. Some people don't mind, but this is the industry standard to ensure an even distributed light and color output.
Hours is only one part of the wear equation. Number of ignitions is another. A search of the web gives more detail as to the mechanisms. Like engine oil, they don't like short trips or start-stop cycling.