The longer you boil hops, the less flavor and aroma you will get, and the more bitterness will be present. Boiling for upwards of 60 minutes will give almost entirely bitterness (which may be out of place in a cider), around 30 minutes will give flavor and some bitterness, and boiling 15 minutes or less will give mostly hop aroma, and only a little bitterness.
Or you could dry-hop by adding directly to the fermenter for about a week or so. That will give you almost entirely fresh hop aroma. Dry-hopping with pellets will also add some sludge and cloudiness to your cider. Hops have natural anti-bacterial properties, so heating isn't necessary.