What is the usual prognosis for ischemia?

Ischemia: Well... Depends where and how :-) ischemia is a technical term for lack of tissue oxygenation, usually caused by a restriction of blood flow to organ/limb etc. Such as a blockage on the supplying blood vessel. But it can also be because of failure of the heart pump or oxygen need of the tissue exceeding supply as in sepsis, high fever... Prognosis pending correction of the underlying cause.

Depends: The question is too vague. Ischemic symptoms are based on location and the extent and duration of the ischemia.