Find My Fitbit app can find a Fitbit up to 100 feet away. Depending on where your Fitbit is hiding, and your surroundings, the distance might be shorter. If there are a lot of objects between you and your Fitbit, the signal will not travel quite as far.
The Bluetooth signal from a Fitbit 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 Fitbit 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.