In March of this year, I booked an Arctic cruise for Aug/Sept 2018. Going with five others that I met this past Dec/Jan on an Antarctic cruise with the same company. We had such an amazing time and got along so well that we were all super motivated to take another trip! That's the furthest out I have ever booked anything, but I do tend to get trips onto my calendar quite early, 6-9 months ahead of time.