My wife and I plan 6-12 month out, and sometimes more. At least for the dates of our vacations. My wife runs a small dog boarding service out of our home, and limits the number of dogs she boards. As a result she has clients that will schedule boarding up to a year in advance. So we need to block out our vacation time early enough to prevent clients from making reservations at those times.
At some point after we block out the time we’ll figure out where we want to go.