Every 6 months is a good idea. Bulbs lose intensity over time and a new bulb replacing a severely overused one can shock the corals by suddenly being much brighter, even at the same wattage. I replace mine every 6 months.
I run my MH bulbs 5 hours a day so I get a year and half out of them. Most of the drop off in MH lamps occurs in the first 3 months. after that the drop off until 18 months is not that much.