Hello, I have talked about this in a few posts but it would be good to have all the info. in one place, thanks for your question.
I assume you mean how long do the bulbs last; the ballast will last for many many years. I should say this is outside my area of expertise (which is growing plants) and you should send an E-mail to Sales@htgsupply.com. Perry the manager will be able to answer your question better than me. However I have used a lot of lights so I will give you my input too.

Most bulbs sold have a “rated life” that is stated on the carton or the literature. It is my understanding that rated life refers to the point at which 50% of the lamps in a large sample will be “burned out". This is not the end of the story, you should know there is a depreciation i.e. lower light output as bulbs age. Another factor to know is that the more you turn on and off your HID the shorter the rated life. You can tell a bulb is done when it starts to cycle on and off when the light should be ‘ON’.

I will assume you are using the HID for flowering and the light is on for only 12 hours a day. If you use the HID for vegetative growth and flowering your lights will have a shorter life then what I suggest.  I would use LED or fluorescent lights for seedlings and young plants and leave those lights on 24 hrs. a day to speed of the growth rate.  When you begin to flower you should know most HPS lamps have a rated life of at least 24,000 hours (over 5 years of flowering). Most MH lamps have a rated life of 10,000-20,000 hours. (2-4 years of flowering). HOWEVER, since all MH and HPS bulbs will slowly “depreciate” (decrease in light output) over time, good growers replace the bulb long before the light output declines significantly. I don't recommend using an HPS bulb more than 2-3 years and an MH more than one year.  I say this because your indoor garden yields are so closely tied to light output, and a 50% decrease in light output may result in a 50% decrease in yields. An HPS lamp will lose approximately 20% of its light output before it reaches its rated life while an MH lamp may lose up to 50% of its light output before it reaches its rated life. A MH will depreciate at a faster rate than an HPS. If your MH bulb is over two years old you may increase yield by 50% just by buying a new bulb. I always recommend keeping one old bulb (the one with the least hours) as a back up. I say this in case you have an accident and you break a bulb, your plants will need light with in 24 hrs, but some of you are 2-3 days from the nearest HTGSupply.com store or receiving a bulb by mail service.

You did not mention LED's but after my posts about growing with LED  I want to include them as a viable choice for growing indoors or in small spaces.  An LED like the Tri-Band has a rated life of 100,000 hours and has almost no depreciation. (That is 22 years flowering). If heat and / or electricity usage are a concern or you are thinking about growing for a long time, you should look at the Tri-Band LED from Htgsupply.com. You could use it in any grow set up but I used it in a grow tent with much success.

Good Growing,
Dr. E.R. Myers

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