30 days in general. But if you have immediate family who live in China, you can apply for a single entry visa with duration of 6 months from Chinese embassy in Bangladesh.

normally its 30 days but if you have your own family living there and they are chinese citizen you can apply for single entry duration 6 months or 1 year multiple 30 days duration. As I am married to my chinese wife I got 1 year Multiple duration 30 days but when I arrive to shanghai I apply for longer visa 6 months I got it its easy only if your married even that now its hard she or me have to have a good work like in a government because private working its hard to prove only for family L visa