A year is what you will hear often, but its possible to go longer. Cider is similar to White wine, which while it will never last as long as its brother red due to the tannin differences, I think they can be good for at least 5 years without having to really try to keep it in a very controlled humid low temp condition. The key is the right sulfite addition, pH, and tannin. Tannins act to preserve wine along with sulfite. While whites have lower pH I believe they use more sulfite to make up for their lack of tannin as lower pH actually keeps more sulfite in the molecular SO2 state. Similar to what would be required for cider. So with that in mind it's worth researching white wine production to see how far you can stretch the life of your cider.