GPU: AMD R9270x
Time : 2:32
Used an add on that comes with Blender that you just need to turn on, and it 'll help you optimize your tile size.
Auto Tile Size
Open User Preferences > Add-ons > Search “Auto Tile” > click the check box > Save User Settings
Now at the bottom of the “Performance” options you should have “Auto Tile Size” just turn it on and it should figure it out for you based on which hardware you have selected at the top.