A murder can be ruled premeditated even if the amount of time before the killing is short.

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Whether a killer acted with the deliberation and premeditation required for first degree murder can only be determined on a case by case basis.

The need for deliberation and premeditation does not mean that the perpetrator must contemplate at length or plan far ahead of the murder.

Time enough to form the conscious intent to kill and then act on it after enough time for a reasonable person to second guess the decision typically suffices.

While this can happen very quickly, deliberation and premeditation must occur before, and not at the same time as, the act of killing. - See more at: http://criminal.findlaw.com/criminal-charges/first-degree-murder-overview.html#sthash.FEEddNYS.dpuf
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http://criminal.findlaw.com/criminal-charges/first-degree-murder-overview.html

I think most people believe premeditation means thinking about it for a relatively long time and carefully planning how it will be done.