Enable "single bounce only" in your volume material - this will make it render MUCH faster. :)
As to render settings, usually what you see inside the Volume Mtl is light cast by other objects, so increasing Light Samples Multiplier might help (from the default 2 to something like 4, but probably no more).
Also note that to get good quality image you need to render about 100 passes, and in some cases even more may be needed.