My stepmother told me that most people who do this sort of thing plan it for 6+ months. I have no idea where she got this statistic, but I'm starting to believe it.
Yann and I started hatching this plan last August. However, we got engaged a couple of weeks later and, instead of planning this trip, started planning our wedding. It was a pretty quick turn-around--about six months almost to the day. Everything was so close together: he gave notice at work in the beginning of February, we married on 2/20, I gave notice at work on his last day, 3/9, my last day of work was the same day that our tenants moved in, 4/9, we left for the Bahamas on 4/15, etc. etc. etc.
Initially, the plan was to start our trip around the end of May. However, we didn't really start planning the logistics until the beginning of March when the excitement of the wedding had died down. By nature, I am a planner, and it's taken everything in me not to want to overengineer this trip. First of all, I don't think there's any way we'd actually stick to an itinerary. Second of all, I think I'd make Yann crazy. Third of all, I am trying to chill out a little bit in my old (wiser, more mature) age.
Despite not wanting to plan the trip, there were logistics that we could not avoid: we had to make a bunch of minor house repairs (Yann and Ky were instrumental on this), pack the house (again, Yann and Ky are saints for taking care of the bulk of this as I finished up work), put the house into storage (ditto), rent the house out, get alternative homeowners' and auto insurance, get a bunch of vaccinations, get health insurance, deal with credit cards and bank accounts, procure visas where necessary for the first six months or so, meet with an attorney to draw up all of the "just in case" documents, get a bunch of prescription medications, and....pack.