They last forever. I moved into my current house over a decade ago and the only bulb I haven't had to change is the single halogen that was already in a bathroom.

With halogen, the lifespan doubles just by reducing the power by a single percent. Having a light fitting far away enough from the DB means that the tiny internal resistance of the wire going to this bulb is enough to make a difference to lifespan.

The light spectrum they produce.