We just finished the Trace last week. Sorry don't remember the time it would take from Jackson to Natchez but its not that long. We got on at Tupelo and I think it took us maybe 6 hours, my guess. In my opinion their is less to see on the lower end. Some places the road was rough but they are working on fixing that.
If you see the plantation homes there be sure to see Longwood, again in my opinion it's the most enjoyable to see. If I can help you with anything else just let me know.