Zero degrees. The ray will leave the two mirrors along a path parallel to the one it came in on. This is the principle of the corner reflector, which is often used as a radar target. Note that the corner reflector uses
three reflecting surfaces (that are set up at 90
o from each other) instead of the two like are being used here.
Wikipedia has a really great drawing that illustrates this two-dimentional problem very nicely. A second link is provided to the article on the corner reflector and the 3-D aspects.
