It will really depend on your light, but anytime mine have been closer to the canopy than about 18" I see signs of light stress, worse as they get closer. I've used an HLG QB and cobshop.net autoCOBs. According to my lux meter (which is much less accurate than a PAR meter, but also much cheaper), this correspond to going past roughly 30-35k lux in 20/4 or 24/0, which works out to a DLI somewhere a bit past 40. That seems like a meaningful limit, because that DLI is also where most recommendations for growing photos I've seen max out; they just reach it via more intensity in 12 hours.
The stress seemingly has nothing to do with heat, it's probably caused by the plant getting more light than it can handle. The symptoms start with the pointed sides of the leaf curling up (very mild), then it tends to look like a magnesium deficiency (pale, between the leaf veins) but localized to the parts of the canopy closest to the light, then the leaves get pale, crispy, and curl up. Damage on light-stressed leaves hasn't recovered when I move the light further, but it stops happening to new growth. If they stretch too close to the light, the symptoms show up again. It took me a couple grows to figure out.
