The hours are posted in the Navigator and vary each day. On our Fantasy cruise there were 2 booking agents--sometimes one was there, sometimes both. The hours do increase later in the cruise as they know the lines get longer then.
Hint--book early. If you know what you want, you can write a note in advance and place it in the drop box OR fill out the inquiry card which is on the desk for that purpose. They will send a quote to your room (no standing in line). If you want it booked, just call the phone number on the quote and tell them to book it to the same credit card you placed on file for the cruise.