2009 HID Headlight Issues

Two possibilities - 1 : Dying bulbs (HIDs frequently flicker, go dim, or turn off when they are about to go bad)
2 : Dying HID Ballast. The ballast is a small box that houses some electronic components. It is mounted to the bottom of the headlight housing. These do not last forever, and moisture intrusion into the headlight housing kills them quickly, as the water runs down to where they are at the bottom. If your headlights get condensation on the inside, then it is most likely the ballasts.

I would start with new bulbs, if the issue persists, it is almost assuredly the ballasts.

 

This is the usual way HID bulbs die. Replace them with genuine Philips brand D4Rs.

 

As Augustus88 said, This is a known symptom of failing bulbs or failing ballasts.
If you plan to DIY search around for youtube videos, etc on the job.

Genuine bulbs are expensive (Sometimes over $100) as are the ballasts.
Be very careful about what you buy so you don't end up with low-quality knockoffs that will only provide a temporary solution.
Start by contacting your dealer parts department to find out the cost of the items.
Then you can search around for other options.
But remember, if the price is too good to be true, avoid at them at all costs.

 

For a long blow by blow report on HID failures in 2006-2009, see Prius Headlight Class Action Update: Settlement Reached | PriusChat