The stew is done when all the vegetables are fork tender. Typically the potatoes will take the longest. Try spearing a potato with a fork around the 50 minute mark, and if it pierces easily, you are good to go. The exact amount of cooking time will depend on how large your vegetable chunks were cut. The gravy will reduce and thicken as the stew cooks and starches are released from the potatoes. It will also continue to thicken as the stew cools.