If you really want to enjoy it, plan at least a couple days. The speed limit is 50 mph AND ENFORCED. At the beginning (or end) of the trail, depending upon which end you start on, pick up the free literature, and especially the map of the Trace that gives all the stops along the way. If you take the time to stop at every spot, it will take longer than 2 days, for sure. The Trace is it's own entertainment. It's all historical, it's not an amusement park ride. But if you enjoy the history of the USA, the "attractions" along the journey will shed some pretty amazing insights into some of the history of the USA. It's an interesting journey, for sure. There again, if you have the time ... take your time, stop at every location... and you might make it to the other end a few weeks later!
Enjoy! It's a beautiful and fascinating historical drive.