2 years on 12/12 HPS bulb

I have had a HPS grow for 2 years now. Should I replace my HPS bulb?

Yes, at least I would

I replace my 400 hps bulb on 12/12 every 6 months.

definitely!!! I read that some people change them every 4-6 months and I was like haha what fucking retards!!! than I pluged in a new bulb next to a bulb that was only 3 or 4 months old and theirs a huge difference... 2 years, probably a gigantor difference... or maybe after a few months they cant get any more dim... hmmm