About as wide as this:

Link to a bigger version of the image.

I snapped the same image with the following lenses:
Canon 15 mm f2.8 fisheye
Canon 16-35 mm f2.8 L USM II @16mm
Canon 24 mm f1.4 L USM II

And marked the reach of the different lenses on the 15 mm image. But I also wanted to see how a Sigma 8 mm circular fisheye did:

Link to a bigger version of the image.

So now you know. That’s the difference between these different lenses on a Canon 5D MkII fullframe camera. And if you don’t like the distortion of the fisheye you can remove it.