Assume 4 degrees C per watt for the LED thermal resistance, then 5 to 10 for the LED to heatsink thermal path. If it's a dropin-style light you've likely got another 4 deg C/W hiding in there. In all, a not-optimized light can have 12-20 C per watt hotter at the LED than on the flashlight surface. If the flashlight is 'really warm' in bare hands, the LED is mostly okay if it's driven under 5W. But over 5W, with gloves on, out of your hands, or uncomfortably warm, the LED is probably close to 150C. At elevated temperatures the LED performance degrades quickly.
If the light goes angry blue, it's far too hot.