dont listen to anyone..haha..bunch of rooks'...

well I've gone 6 crops on a set of bulbs and had plenty of success...and sometimes I get a hair up' and change them after 3 crops.

Best way to know is get a digital light meter and measure lumens for yourself.

They're not cheap...however it will cut out all the bs

I Guess it depends on your Budget..and If you see Diminishing Yield.Locally, I know one Fanatic that gets new Bulbs every Crop..even has a Trade-in Deal with a Local shop!! And the other Extreme is a good Friend who runs his for 18-24months and Braggs about it !! I personally run Hortilux,and Like to switch-out after the 3rd crop.My last change occured after the 4th crop,and I could see the difference.