Some say that impaired kidney function is a contraindication to fasting, but in your own case I would probably have had a kidney function test, prior to beginning a fast.
Also any stones, regardless of their location, are known to soften and pass as a sandy/gravel material, unless that is they are larger than an olive. The length of fast to achieve this would vary from person to person, but I would think you would be looking at at least, 20 to 25 days.
If I were you I would have that kidney-function test beforehand if you are concerned about their health and efficiency, as the kidneys will work overtime if toxemia is present, and harder still if toxemia levels are high.
It is only when toxemia is low enough or absent altogether, that the main healing of the organs will really take effect.