How long are the bulbs run each day? What setting do have the fans on?
The fans should be running at 7.5v minimum. 9v usually is the best setting.
If you have a mixed reef and are properly cooling the bulbs, you can switch them every 12 months. If you have more of a full blown SPS tank, every 10 months.
If they haven't been properly cooled, change every 7 months.