I change my MH every 6 months. Kind of expensive, especially the Radium, but I don't want to skimp. (Radiums are +-$90.00)
I put the first Radium 20k in back in June 08, took it out end of December and put in a 10k Current. Have a new Radium orderd through LFS. I have read that some Coral Farmers will run their frag tanks with a 6700 or 10k bulb for growth then a few weeks before they intend to sell them run 14 or 20k bulbs to color them up. So I am gonna run the 10k until the end of January or middle of February and see if I notice any increase in growth rate.
Tank is getting full and I need something to keep me occupied so playing with the lights seems like an okay thing for right now.