There is no one right answer to how long to cook your packets on the open fire. Every fire burns a little differently, so the skill here is building a fire, and then making sure the coals are burning evenly and not flaring up. Shovel the coals to one side of the fire, creating indirect heat, and arrange the packets on the edge of the heat on a grate over the coals. Cook for about 30 to 45 minutes, rotating often, over indirect heat, until the potatoes are fork-tender and the steak temps at 135 degrees F. This prevents the bottoms of each from burning and ensures even cooking. ​The steak may cook faster than the potatoes. There's no harm in adding it a little later in cook time, especially if you prefer a rare steak.