Tricky choice. There is no set time when they are no longer useful. The light intensity drops steadily throughout the life of the bulb, by up to 40% by the time it finally gives out. You can use it on plants the entire time, but it will still be using the same amount of electricity while it gives out less light. Most people swap out the bulbs at around a third or a half of their rated life, which would be about 5,000 hours, but there is nothing magic about this time. The cost of extra bulbs is more than offset by the savings in electricity by only using the bulbs at their most efficient.

Thanks for the reply! I am thinking about picking up replacements for the indoor season, to have them on hand in case...

From your experience, would you recommend different bulbs (brands and color)? I have heard a bit about Philips Ceramic White MH bulbs (Not sure I have that name right) - any opinion on those vs. the Sunmaster bulbs I am using?

Thanks again!

Tom

I think your talking about the Philips Ceramic Metal Halide. I use it in 400w and love it. Personally I think it's the best HID light on the market right now. It's available in watts up to 400....