how far behind the mirror is the image of an object that is located in front of the mirror

 For a plane mirror , the object distance and the image distance are the same, therefore the image is at same distance behind the mirror as it in front of the mirror.

 For a convex mirror no matter the position of the object the image is always behind the mirror between the focal length and the vertex of the mirror.

 For concave mirror when the object is placed at the infinity the image is formed at F. When the object is beyond the centre of curvature, C,  the image is between C and F. When the object  is at C, the image is also at C.

When object is between C and F, the image is beyond C. When object is at the focus the image is at infinity. When object  is between the pole and the focus the image is behind the mirror.

 Note symbols having their usual meaning.