One day is enough to go Qiandaohu. Hangzhou is only 1.5 hours from the lake. If coming back to Hangzhou the same day, you should shorten your time on several places. The last bus departs at 18:00.

thanks a lot. but do i need to book a tour first?? or i can book there when i reach hangzhou?? about how much to travel to qiandaohu?? any tour to recommend??

If you would like to ease the travel, it is suggested that you participate in a one-day tour group to Qiandaohu when you arrive in Hangzhou. Lots of travel agencies supply the tour and you just have to book the tour one day in advance. Not very sure about the tour accurate price, but should be around 200RMB per person,