It's right around 8 miles to Phantom Ranch, but you can stop and turn around and go back up from any point. But the real question you want to ask is how steep it is and how easy it is to follow and traverse. Compared to every other trail, developed or undeveloped, it is the most maintained, smoothest, best marked and highly traveled trail in the canyon. It is by far the most popular trail, especially for day hiking (which means no overnight stay). It has a lot of traffic, but at least you can get down in there and see the canyon from some point of view other looking down into it. I had an extra day left after a six day trip in, so when I came out, the next day I day hiked the Bright Angel to Phantom Ranch and back up. It did not take long at all. Of course, it is a lot easier coming out when you don't have a full pack, including 2 1/2 gallons of water.
I would also recommend day hiking the Hermit Trail for about 3 miles down (you should have about a gallon of water on you and some form of water treatment process for emergency), but no further. I cannot stress this enough. My wife and I rescued a hiker in distress on this portion of the Hermit back in the eighties. He was suffering from obvious heat exhaustion (he had an empty one quart container clipped to his belt) and required significant rehydration before we carried him back up to the rim and handed him off to park rangers for transport to a hospital.
The Hermit is steeper and not nearly as well maintained as the Bright Angel (but requires no route finding ability in the upper section). It has lots of great views and interesting things to see (I saw a bighorn there, as well as good sized diamondback) and very few, if any, people. If you get far enough to see a long series of switchbacks zig-zagging down from where you are standing, you have reached Breezy Point (3 miles from the rim) and are looking at the Cathedral Stairs. Here you need to turn around and go back up. The Cathedral Stairs can be brutal, especially when all lit up in the heat of the day, and they comprise the steepest portion of the entire trail.
Also, the aptly named Grandview Trail provides stunning vistas, but it is hella steep, with a 2,500 foot grade change from the rim to Horseshoe Mesa (a distance of 3 miles). Again, carry adequate water with you along with an emergency means of water purification for emergencies. With the Grandview especially, allow twice the time to come back up as you took getting to where you decide to turn around.
Both of these two alternative day hikes I have suggested would require 6-8 hours of hiking to complete a round trip from rim to turnaround and back to the rim, depending on your age and general condition. They both are best done in real hiking boots as well. There are some serious rough spots in these two trails and ankle support can be a lifesaver, literally.
The hours I spent in the canyon are some of the most treasured of my life. It is a mystical spiritual experience that cannot be described or explained. I never felt more alone or scared or alive than the seven day solo I took down the New Hance. It cleansed my soul.