Hrm...
I'd say it's a routine Ambush roll, the results are read on the chart and the RR is a Stamina roll to survive.
I'd say it takes one standard round to complete provided the attacker is free to move, has a piercing or slashing weapon, and is not under attack. If any of these conditions are not met, the coup de grace can't occur.
I'd use the same mechanics when attacking a sleeping or otherwise helpless defender.
>>ReaperWolf