let me know the earliest, latest and preferable timings

The Li River cruise usually departs from Guilin Zhujiang dock at 09:00 or 09:30 every morning. After about 4.5 hours, you will reach Yangshuo at around 14:00. It takes around 50 minutes driving from Guilin downtown to the Zhujiang dock if no traffic jam. So you should prepare enough time for the driving.