I've let it go hours after hitting cancel. Admittedly, 90% of the time I hit cancel because it's incorrectly rendering with the CPU instead of the GPU, and consequently the render itself would take many hours, but the point of hitting cancel is to stop so I can do it correctly and not waste all that time..... So I end up just killing the process in the task manager, and then restarting Poser, but that's not optimal.