They can book it starting 180 days out. I think you can contact them even earlier than that and they will put your name on the list and sign you up when the booking window opens.
We did it last week, and we called concierge more than 180 days before to let them know what we wanted. They in turn called us at the 180 day mark, and we reserved at that time. We did the same for the Sunset safari. BTW, you can reserve 180 days from your check IN date, so it could be more than 180 days from the date of your safari. Sunrise safris are on Sunday and Thursday, so you would need to be staying Saturday night or Wednesday night to do it.
One thing that is a bit confusing for the safaris (both sunrise and sunset) is that you must give them your CC number to reserve it, but they do not charge your account until you do it. So be prepared for the big bill at the end of the evening on the sunset safari and don't forget that DVC members get an additional discount on the sunrise safari. Sunrise is $60, but DVC members get it for $55.
I dont think that is right as I have a Reservation at AKV not through MS (I am on waitlist though) Got sent an itineray planing sheet from them last week and it siad the price is $55 adults and $27.50 kids. And they have no idea that i am a DVC member. And just so you all know the meal is now at pizzafari not tusker house.