If you have only…
One day in Yangshuo
Li and Yulong rivers are about enjoying and relaxing in superb nature, combined with moderate physical effort. However, if you are really short with time you can combine Li River and Yulong River in one day. This day wouldn’t be so relaxing, you would have to rush, but if you don’t have other option, it is still worth doing. Of course, this has to be one full day.
Two days in Yangshuo
On the first day visit Li and Yulong rivers as described above in One day in Yangshuo. On the second day do Day 3.