I have a galaxy hydro 300 watt LED. The manufacturer recommended distance is 24 inches in flower. I have just recently read an ad from a different manufacturer that said 8 inches from a 600 watt system … My question is… … When do LEDs burn ???
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Here’s a couple of Roberts blogs on this issue:
and here:
"Distances can vary-
Because LED lights are not as standardized as other types of lights, it is crucial that you check the manufacturer’s instructions for how far away from your plants your new lights should be. This distance varies according to the bulb size as well as the type of lens, so don’t start using it until you feel confident that you fully understand what should be done with your specific LED lights.
Generally speaking, smaller LEDs that have 1W chips should be a minimum of one foot away. The majority of LED lights with chips that are 3W or larger, however, should be at least 1.5 feet away. Otherwise, you could give your plants light burn. Remember that you can always ask the manufacturer for recommendations yourself. If your plants are starting to appear burnt on just their top part, you should probably try moving your LEDs further away."
Hope this helps,
Dumme
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I had my 300w led about a foot away and noticed some curling in the leaves, moved it up to 2ft and they’ve flattened out. I’d say go with what makes you comfortable and adjust accordingly 
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Rule of thumb: put the back of your hand under the light. The distance where it is not burning you is the minimum safe distance.
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I agree with @Aquaponic_Dumme to extent. The higher output led’s will have more light intensity. The problem is taking a manufacturers word for it. A 3 watt led is an led that has fv of 3v and max current of 1000ma. A 5 watt led has fv of about 3.2v and max current of 1500ma. And a 10 watt led has fv of 3.3v and max current of 3000ma. So any of the three diodes run at say 700ma have pretty much the same output. The only real difference would be a very efficient (expensive) led vs a less efficient (cheaper) led.
I can guarantee you that not a single light manufacturer runs their diodes at max current. It requires too much cooling and drastically reduces the life expectancy. So when you see on amazon that a light has a hundred 10 watt diodes, you can pretty much throw that information out the window. The reality is they’re probably only driven at 700-1500ma, and you can expect about 25-50% of that in true output.
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@dbrn32 is right on the money. I have 1,000 watt running at 22%, 400 watt at 49% (COB), and 300 watt running at 38% of “rated” output. To make things even more confusing, some compare wattage at the front end to MH/HPS lights. Not sure where that came from. Basically, if you can comfortably hold your hand there your plant can be there.
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I appreciate the input . My light of choice is the great, 600 watt HPS.dimmable. But I battle with HEAT…in my particular grow situation. That’s one reason to love LEDs. I find that the led is a bit slower in flower but produces more trichrome .
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I check the top of my plants a lot for any signs of burn. I also do as suggested by the previous guys and put my hand on the plants top to see if I can feel the heat. I want them to get as much as they can handle without burning. I think it takes a little adjusting/practice to learn what is best for your plant. Thanks @Aquaponic_Dumme for linking those articles. I’m going to give them a read.