I start preparing months in advance. I'm that nerd who considers cruise prep part of the cruise, and I like to spread it out over several weekends. One weekend, I'll pack my medical/ first-aid kit (and, yes, I bring an entire pharmacy with me). One weekend, I'll pack toiletry cases. You get the picture. I hope I never become so jaded I stop getting pleasure from prepping for a cruise.
As far as actually putting everything in suitcases, I do that two weeks in advance. The last weekend is reserved for post-cruise prep like tidying up, cleaning out the fridge (aka- eating all the leftovers from the last 10 days), making sure all the laundry is done, and scheduling any household maintenance that we've been putting off to be done while the ferocious lap dog is at the kennel.