I'm thinking about 1870-ish.
She isn't wearing a wedding ring, (but then neither am I) and she doesn't have an elaborate widow's cap, so maybe not lost her husband. However, she certianly is in deep mourning.
The short jacket under her cape and the fringing sugest 70-ish. No hoop, so not much earlier. Can't tell about a bustle, but the skirt is full enough....also the deep band on the bottom of the skirt is about that time, so not much later than that.
Hope this helps,
Leonie.