When do cruises open for booking?

DH and I are wanting to book a Galapagos cruise on Xperience for December 2019. I see as of today only cruises through June 2019 are accepting bookings. Can anyone tell me if cruises open on a month-by-month basis (as in cruises for July 2019 are opened on Sept 1 2017, Aug 2019 on Oct 1 2017, etc.) or if they open several months at a time on a regular schedule (like every quarter they open three more months out or something like that)? Since Xperience is a small ship and this is a once-in-a-lifetime trip for us (celebrating our 10th anniversary, my 50th bday and DH's 60th bday), I want to be able to pick my cabin as soon as the cruise is open for booking.

They have been released in November and December, in past years. They do not roll out month by month. Start watching in November and also check back here on Cruise Critic. when anyone here has knowledge of which cruises are scheduled to be announced in which week, they usually post it here.

You have a long wait they don't open month to month.

Would the onboard booking people know which cruises will be offered for the fall of 2019

They do not have any "insider information". They don't know any earlier than any of us.