Hello,
The best time i managed to pull off was 2:09 min with 512x512 tiles with GPU CUDA render on a nVidia GTX 960
I just found a checkbox under the tile size settings that says Progressive Refine. Turns out that it renders the whole image really fast with a lot of noise, then refines it bit by bit. I got a very rough idea of what the image will look like in about 35 seconds.
This is an image after 18 seconds of the render fiddling with it:
Best of luck,
Svetlin