How do you know when to change MH bulb?

[danmhippo]

I have heard people saying MH should be replaced every half year. But what is the determining factor that we should look for? I am sure not all bulbs made the same, some brand of bulb has longer life, yet some are sooner to lose the designed spectrum. How or what do you look for to determin the MH bulb has reached the end of its useful life?