Bulb replacement

bulb=lamp
It depends on what type of lamp you're using.
These recommendations are based on 18 hours of daily use.

Type When to replace

1000w (mh) lamps 12 months
1000w (mh) lamps "agro sun & warm deluxe 15 months
1000w (mh) lamps u cool deluxe 6500k 12 months
400w (mh) lamps 18 months
400w (mh) lamps u cool deluxe 6500k 15 months
250w (mh) lamps u cool deluxe 5400k 12 months
175w (mh) lamps daylight bulb 5500k 15 months
All regular 75w to 250w (mh) lamps 15 months

Replace all hps after 24 months, with the exception of the son agro and super agro, replace after 15 months. Fluorescents should be replaced every 15 months. After 12 months, check the tubes if running more than one tube on a single ballast. Always replace all fluorescent tubes at the same time. Remember to always write down the day that you start using a new HID lamp. This will allow you to calculate when to replace it for best results.

I read some interesting info on this, but I can't find it right now.
I wish I could though.
I seem to remember not to use a bulb for more than 3 grows, than replace it, and keep it for a backup.
There were two valid points, one was lost lumens, the other was a claim that your electric cost goes up as the bulb gets older.

Two good points about the lost lumens and electic cost going up. Also a good idea to keep the old bulb as a back up. You never know when something might go wrong. We've had several posts from desperate people had wished they had an old bulb sitting in the cupboard.