When the plants are dried whole, in the dark without much if any damage, the beginning of the drying time is the dark period at the end of flowering. There is some of this effect even when colas are separated or leaves are chopped prior to drying. Because of this, no extra dark period is necessary to get the most from your plants. This doesn't include changes to the light schedule in the last couple weeks, other than total darkness for the remaining flowering time.
So, out of the two choices, our answer to your original question would be 24hrs.