Update: I tried setting the time limit to 10 seconds. When the countdown completes, the frame buffer is automatically cleared and it starts rendering from scratch all over again in an infinite loop. I also timed how long it takes to finally stop when pressing cancel; it's 11 seconds, meaning that is how long I have to wait each time I want to stop the render so I can tweak some settings.
I created a new scene with just a cube and nothing else which works fine, so it must be something specific with the test scene I was using. All that's in it is a long trench 8 meters wide, 6 meters tall, and 5 kilometers long. The side walls have the greeble modifier applied and there is a corona sun and target camera in this scene as well. Oh and one basic corona material applied to it all. Total polycount is 894,542 quads.
