Bright Angel Trail is a 7.8-mile one-way descent from the South Rim to River Resthouse, dropping about 4,400 feet through the canyon's rock layers. The trail is maintained and easy to follow, but this is a serious point-to-point hike, not a loop. Allow about four to five hours downhill. For a day hike, consider an earlier turnaround: the first resthouse makes a 3-mile round trip, Three-Mile Resthouse makes it 6 miles, and Havasupai Gardens makes it 9 miles. Each is a shorter out-and-back; the map continues to the river.
Do not plan to hike from the rim to the river and back in one day. That would mean roughly 15.6 miles and both descending and climbing about 4,400 feet. If you're hiking only partway, remember that every mile down becomes a mile back up, and the climb out takes much longer than the descent.