The risk is immediate. The 5 mW 'safe' limit is only because the blink reflex will shut your eye before damage is done. So the anwer here would be 20 times faster then you can close your eye, which equates to something in the order of 10 miliseconds.
Obviously there is a rather large safety margin between what is generally considered safe, and what actually does damage every time it happens. But crossing that margin 20 fold would surely be a true hazard.
Non-reflective, matte, surfaces would be quite safe, as they re-radiate the laser energie as if they were point sources. At any distances that greatly diminishes the risk.
Reflective (or glossy) surfaces can be as bad as a direct hit if they are mirrors. A bouce off a glass pane is normally about 4% at a normal angle, but much more at shallow angles, so still possibly very dangerous.
Blu-ray warrants extra caution: A beam that has enough energy to do serious damage does not appear very bright, and can be slower in triggering blink reflex, just because 405 nm is borderline visible light. 10 mW of 405 is just a very dull spot on a non-fluorescent surface, but at least as dangerous as a rather blinding 10 mW of 532.
