From Wikipedia... State laws in the United States vary as to definitions of "premeditation." In some states, premeditation may be construed as taking place mere seconds before the murder. Premeditated murder is one of the most serious forms of homicide, and is punished more severely than manslaughter or other types of murder - often with a life sentence without the possibility of parole, or in some countries, the death penalty .
I know in Oklahoma it can take only seconds. I was a jury foreman and still have the jury instructions in a murder trial.