RAT 5:
The engines would need five seconds or more to produce significant thrust
I can't remember from my CF-6 days, but on CFM-56's I used to suggest a min of 40% N2 on finals, for instant response, but if idle was needed to ensure that you had 40% when your speed was target +5kts at the latest, again to give instant response to freeze you at target speed when it was reached.
I submit you probably mean 40% N1 (fan rpm).
Core speed N2 at ground idle won't be less than about 60% for either CF6 or CFM56.