Possible Clues:

• Two weeks
• Sounds too weak when it's dark in the castle
• 'Keep it dark then, when it's short of 12 across (9)'
• About half a month
• Period given to RN formation to enter battle
• When it's dark for the keep for the half of 1 across
• By the sound of it, too weak to be too dark under the castle
• Badly torn in struggle that lasts some time
• Period given to R.N. formation to enter battle
• That sounds too weak for the castle overnight