How long will it take to close premolar extraction gaps?

I had 2 lower premolars extracted 8 weeks ago - and have had self-ligating braces with power chains for 6 weeks now to close the gaps, in preparation for double jaw surgery. How long does it typically take for these gaps to close? When would be reasonable to expect to have the surgery? I am using Acceledent daily as well. I feel my gaps have maybe closed 1mm so far, which seems very slow. Is there a faster way/technique to close these gaps?