How long do headlight bulbs last on a Toyota Prius?

My original bulbs lasted 4 1/2 years. Then I made the mistake of using the most expensive bulbs because they should have been the best right? Wrong! They only lasted 3 months. When I went back to the least expensive - let's just put it this way- I went through 3 sets of the expensive ones in 7 months. Since I have put the least expensive bulbs in - I am going on 18 months without having to replace the bulbs. (be aware they usually blow out in pairs)