People who have light amnesia due to less severe head trauma or other head injuries will usually regain most and sometimes all of their memory within a few months to a year.
In more severe cases of amnesia and head trauma the person who suffered the major head trauma or amnesia may not get any of their memory back for the rest of their life.
When the head trauma is severe and causes severe amnesia then it's usually not curable and the person cannot remember much of anything before the amnesia.
Still sometimes the person may get their memory back if they are lucky but if it's severe amnesia the changes of regaining full memory is slime to none.