what is a good time level to spend in the area to enjoy Li river and local attractions

The best time for visiting there is from April to October.
For GL city, I think you can spend two days. Then go to Yangshuo by ferry and stay there that night. Then the next day can be spent around Yangshuo and you can go to Yulong River. In the afternoon, you can go back to GL. Then you can plan to see the terrace in about two days and th Daxu Ancient Town in one day.