Just to add to this... many of you know me, but there are many names I haven't seen. So this is info from the past.
The reason why MH bulbs are recommended to be replaced every 10 - 12 months is because of the color spectrum. Take a 10K bulb for example. When they are new, the color spectrum will have peak spectrum around the blue range. As the bulb ages, the blue spectrum peak reduces and the yellow to red spectrum starts to rise. Eventually, the 10K bulb will have peak ranges in the yellow to red spectrum, which is ideal for algae growth, as the sun's color spectrum is more towards yellow to red, or your 6500K bulbs. Aside from the color shift, the bulb intensity also reduces with age.
The bulb will run for years without going out, and for most tanks, especially ones with SPS, you will notice change in coloration and polyp extension.
To your eyes, they will look fine, but under a spectrometer, it will look different. As a previous SPS grower, I replaced my MH bulbs every 10 - 12 months. Many serious SPS keepers also replace their bulbs after 10 - 12 months as well, because of the color spectrum shift.
Not sure if the article is still on Reef Central, but search for it.
Cheers,
Jim