Find My Headphones app can find a device up to 100 feet away. Depending on where your headphones are hiding, and your surroundings, the distance might be shorter.
The signal from a device can travel through walls, floors, and ceilings. This means you might get a warm or hot signal in one room in your house, but your lost device could be in the room right next to it, above it, or below it. Look in all locations to your left, right, up, and down when the indicator says warm or hot.