Flash duration is the length of time a flash is generating light. The flash duration is typically very short, often between 1/500sec and 1/1000sec, but often even faster than that. This is useful for freezing fast-moving objects, but it can make exposure settings, particularly the shutter speed, more complicated. Almost all cameras have a maximum flash sync speed which is much lower than the actual flash duration. Many flashguns offer a high speed sync mode to get round this.
Flash duration depends on the model, its power setting and sometimes the mode it’s being used in.