I don't know that there is a general rule, but most LED retailers tell people that the lights need to be 18 to 24 inches from the plants. That said, a whole lot of people seem to go closer than that and get away with it without burning their plants. It's pretty important though when the plants are just seedlings to keep the LEDs a good distance away or you can burn the heck out of them when they are babies. Most people do some experimenting to find out how close they can keep the lights without burning plants. The distance away is usually relative to how many watts the light is.
Plants are pretty smart. They'll definitely find a way to tell you when the lights are too close. The smaller 50 and 100 watt LED bulbs that are often used for side lighting can be placed fairly close to your plants with no problem at all. The smaller LED bulbs don't seem to burn leaves unless a plant is actually touching the bulbs, or at least that's been my experience.
I really like LED lighting, but when your plants are little, they just love CFLs. You can include LED lighting at a maximum distance from the babes, but place your smaller CFLs just inches away from your seedlings. Out of all the different kinds of lighting, I think the cool white spectrum CFLs give baby plants the very best start. But once they get into flowing age, it's the LEDs that's going to help pack on the weight and make a much denser bud than you would normally get with most CFLs.