There are many sprite sheets where the bounding box of each frame is essentially always the same size (e.g. tileset), with such an image you just have to figure out the size once and then multiply it accordingly. With more complex sheets as you've shown above one would at the point of packing all the standalone images into one sheet, let the packer (see an example above) generate a accompanying file that would tell where which sprite can be loaded from etc.
Of course if you just end up with the sprite sheet and the bounding boxes are irregular in size, then there's not much you can do other than map it manually. Maybe, just maybe there is some application that would be able to find out the borders of a character and thus making things a bit easier, I don't know such an application though.
