How often should I change my hps bulb?

OK never thought this was a big deal til... I have gotten two harvests that were underweight in my opinion. How often do you guys change out your bulbs? Resin production is killer, quality is high, yield is low.

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Sorry, I am on my phone doing this, looks like I posted it twice somehow

ive been using same 600w hps for almost a year now. still pull heavy yields. i'll be changing at the year mark. have read that the bulbs that claim are super high powered and shit arent all that. i'm using the bulb that came with my ipower and apollo setup. no problems here

I do every 9 months to a year.

2 years

And do you ever notice a decrease in yield during the last months before you change it? I'm using really old gear...

what is really old gear? and how many hours does the bulb have?