How long do you run them everyday? and what ballasts? Those answers will determine how often you should change your bulbs, roughly. FYI, I am running my phoenix 250W 14K on e-ballasts 6 hours a day. I change them once a year.
Here are a couple articles in my bookmark that you may find useful:
http://reefkeeping.com/issues/2007-04/ac/index.php
http://kb.marinedepot.com/article.aspx?id=10857