Rule No. 1 for a full descent into Grand Canyon is to cast off the idea of making the journey down and back in one day. While people have survived this foolhardy attempt, it's strongly discouraged by experts and by the Parks Service itself. Arrange for a stay at the bottom, reserving space in the Campgrounds or at Phantom Ranch.
In fact, it's strongly suggested that you plan for at least a two-day sojourn at the bottom of the canyon: one day to recover from the arduous journey down, and another to actually enjoy the experience of being somewhere so few people will ever see. There's a lot to enjoy on the canyon floor, and you'll need all your wits and strength to make the trip back up.
For a hike of any length down into the canyon, you should assume the return will take twice as long. Even experienced hikers can find this reversal of priorities a little hard to get their heads around, so take that rule of thumb very seriously, and think hard about what it means for your schedule.
As for the best route, it's generally suggested that South Rim hikers take the harsher South Kaibab Trail down into the Canyon, and Bright Angel on your return. (North Rim hikers only have the one main option, so you're weighing the possibility of a less crowded adventure against the more narrow set of options returning.) For any trail, hikers find it takes about 4 to 5 hours on the descent, and up to 8 hours average getting back to the rim -- but again, the challenging nature of the journey renders those averages much less important in your plans than the overall time the full trip will take, including rest periods.