how any grows does a bulb last?

[greenguy]

how many grows does a bulb last? say a 1000w HPS using it 12/12 for 2 months per grow...

 

[leafminer]

Efficiency gradually declines with hours. But you should get quite a few grows. At least five grows before I'd think about replacement.
In my case the answer is usually "until it has an accident". The arc tube is very fragile, easily fractured with a bump even though the outer glass isn't broken.