HID bulbs may illuminate for years & for general lighting applications that may be OK, but not for vigorous plant growth. The best way to know when to replace a bulb is to check the light output periodically with a light meter.
Otherwise use the rule of thumb - If the bulb is used for 12 hours+ a day, replace every 12 months.
The proper burning positions for bulbs
BU = base up/HOR = horizontal/U = universal
We no longer list the standard lower output Metal Halides (MH). Call for pricing. The bulbs we offer are the HIGHEST OUTPUT models of each class.