My GF and I started planning our most recent (July/August) trip in March. We went through several iterations and ended up with a much better schedule than we would have had if we had gone with our original concept. We also had enough lead time to book accomodations during the one part of our trip that hit a local travel peak (Edinburgh at the start of the Fringe). We've done other trips on the spur of the moment, but none that were as complex.

We're sold on the benefits of advance planning, in large part because of the improvement that comes from mulling things over and talking about options. Hard to do that without lead time.