How often do you change your HID bulb?

Common convention typically dictates that one replaces a light bulb after it fails. I've heard a few ways people manage their HID lamps that defy that convention. So it's poll time!
Please discuss below

havent had to yet, but ill change it every 18 months i reckon, none of those options really applies...

agreed i picked the first one die to i use mine for 3 crops max

As I understand it, a light bulb of any kind starts losing it's intensity ( can be measured in lumens or candles with a meter) WITHIN hours of it's first use. So if it's a bulb on your nightstand, you probably won't care and may not notice the bulb dimming before it fails. I need a light meter!
I just was hoping for a good section of indoor folks to throw their two cents in!