Here is an interesting article on bulb life using the Phoenix bulbs as the test subject
http://reefkeeping.com/issues/2007-04/ac/index.php. This guy claims a Metal halide bulb loses about 25% of its output at three to four months and at 18 months it is still at 70% of its initial output. So if a person is not going to change their bulbs every 4 months they are just as well to leave them in for a year and a half.