Enfodesk published the report of Chinese Weibo access statistics by hour as of Oct,2011.
This report targets each Sina(新浪微博), Tencent(腾讯微博), Sohu(搜狐微博), Wangyi(网易微博) Weibo service.
The peak time of access number is around from a.m.8:00 to p.m. 4:00 in each Weibo. This is a working time ;-). On the other hand, the access number down at night time.
If you are marketing person who are targeting China market via each Weibo, it's better tweet on there daily time of UTC+8 :-)
Original article => Enfodesk