Sorry to hear about this situation.
An ischemic foot itself will not cause death.
If the foot becomes necrotic (infarcted) -- then that is when the skin and tissues break down, become black, and allow infections to take hold. This is called "gangrene", and the treatment for this is generally amputation to prevent serious infections from occurring. If this occurs, and is not treated, then it could be weeks, depending on the condition of the foot.