When to change BULBS?

When exactly do you know to swap out a old bulb for a new one?

I currently am running HiLux GRO 1000w by Ushio and it's awesome but when do u swap em out?

When they won't turn on anymore. Or one year.

Some people say every 2 grows or every year, I am assuming it is being used for a 12-12. I was wondering the same thing and looked it up a bit ago and the info I received is that a HPS will lose about 20% of its lux in a year and a MH will lose about 40%, so I would say every year and I have herd of some ppl using the old bulbs for side lighting.