For all images a threshold of 0.0005 was considered
The following images were generated without using ray differentials
Sample Count: min=4, max=64. Render time 1:27
Sample Count: min=8, max=16. Render time 1:28
Sample Count: min=4, max=32. Render time 1:05
Sample Count: min=8, max=64. Render time 2:00
With ray differentials I found that for any sample count above 8 no noticeable differences were made on the image. The following render took 2:05 to generate with min=8and max=64. A comparable image with min=4 and max=8 took 41seconds
Sample Count: min=8, max=64. Render time 2:05
Specs
Processor: Inter(R) Core i7-35337 CPU @ 2.00Ghz
RAM 8.00GB
NVIDIA GeForce GT 740M