If I'm going somewhere out of easy driving distance and hoping maybe some friends will come along, I'll start floating the idea about a year out, start making more solid plans about 6 months out. Give people time to get passports, save up for plane tickets, etc.
If it's an overnight to a long weekend with just me and my wife, maybe a couple weeks, we could probably do day-of sometimes but we gotta make sure someone can watch our dog.