I know that a host the size of the K40 will allow that kind of dissipation. I have a modded TN31 with a 5mm*31mm copper heatsink that I made myself that will run for a while before getting too hot to handle. The topic lead me to believe that we were discussing a single 18650 host not a 3*18650 host, which based on your estimation of 15 minutes with a host that size would be pretty difficult to pull. My TN31 can run untill the batteries run out on Hi in any conditions less than 110F and still be held, but it has a massive copper heat-sink and is far from stock in relation to heat-sinking to the body of the light.
Yes, you are certainly correct that single-18650 lights are not going to run very long with an over-driven LED. But there wasn't any mention of size or configuration in the post I quoted...now that we have clarified what we are talking about, I think the situation is clear and we are in agreement.