The quote from Nichol Kessinger comes at 3:21:04 in the clip below. It’s from Kessinger’s second interview, close to the end.
Kessinger adds that over the final weekend, Watts also informed her that they were putting the house up for sale [something she adds a few seconds after the first quote].
What this clearly indicates is when Watts returned from North Carolina on the evening of August 7th, he knew then what he intended to do. It seems clear then that the premeditation was in place at least 1 week prior to the murders, but given Watts’ standoffishness in North Carolina with Shan’ann, it possibly started during the first week of August while in North Carolina, when he was away from Kessinger.