To kill with malice aforethought means to kill either deliberately and intentionally or recklessly with extreme disregard for human life.
Premeditation means with planning or deliberation.
The amount of time needed for premeditation of a killing depends on the person and the circumstances. It must be long enough, after forming the intent to kill, for the killer to have been fully conscious of the intent and to have considered the killing. Penal Code S 189.