Headlight problem--frequent failure (not the winking problem)

I have an '09 Prius with 105K miles which I bought new. It has standard (halogen?) headlights. For about two years now bulbs fail after about 6 months of use, first one side, then maybe 2-3 months later the other side. Is this a known issue or symptom? Replacement isn't expensive, but it sure is aggravating to have to do this 3-4 times a year.

Any advice greatly appreciated!