Are you talking hours of operation Ed? or are you talking about when you should change out the bulbs?
I have a hard time with these things because I always see blanket statements of every 6-8 months for halides, every 8-12 months for VHO and some other figure for PC. To me this is rediculous. If you run your MH 3 hours a day vs. someone who runs them 12 hours a day you don't need to change them as often.
To my understanding, there are 2 reliable ways to know when to change the lamps:
1) get a meter that measures spectrum and change the bulb when you see a shift to a non-useful spectrum
2) watch your tank and when you start to see the changes in your inhabitants (i.e. increased nuisance algae, change in coral color, decrease in growth rate).
Just my 2 cents worth. I'm sure there will be many that dissagree with me on this, but without the correct meter I think you are either too early or too late when changing on a pure schedule basis.
Ed its really tough to answer that as thier are a ton of variables. Every bulb will be different on every different ballast, the how long to you leave them on during the day, then is the bulb being overdriven or underdriven and so on. thier would be a buch of different answers for every bulb.
Sorry, for such a broad statement.
I remember running VHOs for 12hours and Now I run My Halides for 10. I remember my VHOs with Icecap electronic Ballast going from one spectrum to the next pretty fast. I remember my VHOs lasting about 4-6 months. It has been hard for me with My Halides to notice the difference. I am running two champion 250 watt ballast with two single moguls. Yet I know that they too are burning from one spectrum to the next.
according a series of tests by Joe Burger, MH's will actually last quite a bit longer than 12 months.
In this link Joe, (JBNY) goes through the whole testing of lots of different bulbs and ballast combinations in order to identify PAR levels of various combinations. The surprise of the test, (to me anyway) was how well the XM 10k bulbs do. Joe mentions (somewhere, sorry don't know exactly where...pretty far along though) Joe mentions that the manufacturers bulb life reccomendations are close to 7-8 thousand hours...which comes out to about 18 months at 12 twelve hours aday......things that make ya go Hmmm......