It really depends, actually.
For this October trip, there are certain things we started packing way back in February.
It's a Halloween Party trip, so whenever we get bits of our costumes together, we stick them in a "Disney Bag". We've got our Brita Bottle and filters in the bag, as well as all our passes and reservation papers.
I just picked up new travel toothbrushes the other day (we always get the electric kind, I found some that are rechargeable on clearance for $4 each) so into the bag went they.
We always have a toiletry bag packed with shampoo, conditioner, toothpaste, deodorant etc, so don't have to worry about that, except of course on the few times when we forget to put the bag itself into the suitcase.
Around 10 days out, I'll start doing laundry and pick out the week's worth of clothing, then pack the suitcases and be done with it. Whatever clothing is packed is off limits before we leave.
So anyway, I'd say that 10 days is really when we "pack", but the majority of our "last minute stuff" is together way before that.
As for how many outfits, generally one per day is enough (if you dress the way we do, which is "neat.") I've seen some people dress REALLY sloppy and I certainly would not want to sit next to them in a restaurant, and I'm sure that's why Disney put that new clothing policy into effect. I'm only 28 but, man, I don't know where these people got their habits from. Yeah, I wear belly shirts and hip jeans sometimes, but nothing that exposes my private bits and I would certainly never wear them to a restaurant. (And don't even let me get started on the whole "anyone who has a belly shouldn't wear a belly shirt" argument.)
If you like to wear hotpants and wifebeaters though, I'd say bring day clothes and a different outfit for dinner each day.
Also, if you're going to go to the water parks or on rides that potentially will get you wet, make sure you bring extra sets of clothes (including underwear) for those days. Kali River Rapids catches people by surprise.
And always pack a sweater or a jacket. People don't think of this, but Florida nights can get chilly. And the air conditioning can get FREEZING in the restaurants.