I don't put anything in a suitcase until the night before, or the morning of. But I do start piling things in advance. I will put beach towels, swim suits, etc, in a pile a week or so before a trip, when I think about it. If there are clothes that I want my kids to take, I will pile those up ahead of time, if they have to dress up or something. If I know I want to take a particular clothing item, that I won't be wearing before the trip, I put it in the pile.
The night before the trip, or morning of, I pack my own stuff, tell each kid, "Bring me 8 ****s, 6 bottoms, 6 undies, etc" and then pack theirs, or have them do it. If we are flying, I put more than one person's stuff in one suitcase, so its easier for me to do it.
It doesn't take long to pack, IF the clothes are clean.