This depends on a few factors: when you want to travel, how flexible your dates are, how big your group is, and how picky you are about your itinerary & accommodations.
When do you want to travel?
The busiest times in the Galapagos are around Christmas and New Years, followed by Easter and Spring Breaks, and then the summer holiday season. If planning to travel over Christmas, you will need to book your trip over a year in advance.
If you want to go in April or May, you should book your trip 9 months or more in advance, as these months are also very popular for the warm weather and calm seas.
The other seasons don’t sell out so rapidly, but the earlier you can plan your trip, the more options you will have available to you. Generally you should think about booking your trip between 3 to 9 months in advance.
How flexible are your travel dates?
If you only have a week long window in which to visit the Galapagos, you will have really limited options. You will want to plan your trip well in advance to maximize your options. If you have a month in which to travel, you can wait until a bit closer to your dates, to see if any deals come up. I would recommend 3 to 6 months in advance.
How big is the group that you’re traveling with?
If you’re traveling solo, or there is a group of 2 or 3 travelers, it is pretty easy to find space at almost any time, except for over the holidays. If you are in a group of 4 or 6, it can be more tricky, especially over the busy seasons of the holidays, and you will want to plan 8 months to a year in advance. If you are in a group of 7 or more people, you will need to plan a year or more in advance.
How picky are you about your itinerary & accommodations?
If you are very particular about which islands you want to visit, how long you want your trip to be, and what sort of accommodations you want, you should plan your trip 6 months or more in advance.
If you don’t want to see any particular islands, you’re flexible on trip length, and you don’t really care what sort of boat or hotels you are in, you can pretty much plan your trip any time. And there are some great last minute deals out there!