I'm no expert, but with a 15W power draw the answer to the "how long" question is likely to be "perpetually" with active cooling. Now, Apple tends to let its processors run very hot before throttling (I had an old i5 MacBook which easily hit 98C), but again, a 15W power draw means there isn't all that much energy to push into whatever heat sink or cooling solution they want to use. (In the real world it's rather more complicated, with different load intensity on different parts of the SoC, but still)