To change or not to change that is the question.

I have heard people say they change their HPS/MH bulbs every 6 months others every year then some say every two ,three or four grows. I guess the best way would be to use a light meter and change when the output drops below acceptable level. I would like to here from all you experienced farmers how often you change your bulbs and why. I am planing on changing every 3 grows.
BTW guy at hydro shop says every 6 months but he is also selling the bulbs.

I have always heard the same thing and just recently learned it first hand bought 2 new bulbs about a year ago and have had about 7 crops from veg to flower under these 2 1000 hps
and a freak accident happen where I bought a new one same kind and brand and hook it up and a HUGE difference between the 2
so I will be getting a new one and replaceing them as the budget allows hopefully every 6 months or so
and probbally selling the old ones on ebay as used bulbs to get some money back

it all depends on how many light hours u use ,but your bulbs will tell u how many light hours u will get outa them , i would change roughly every 6-8 months