From experience, some hotels will have only availability up to 1 year, others only load rack rate but with availability over 1 year time-frame.
If you need to book a great number of rooms (say for a wedding), I would suggest you liaise directly with the property and they will happily book the rooms for you.
If only planning your holidays, booking so far out is probably not going to get you the best deal as discounted rates and promotions tends to be loaded within the one year time-frame (if lucky).