I believe you only have to dump until the plane can touch down at it's maximum permissible landing weight. Not sure what that is for an A380 or how much more heavy it would have been than that at take off from SFO. I think planes dump at quite a decent rate. I googled but came up short. That said Boeing have an article that explains you can land "overweight" if needed. I would imagine the same is true of Airbus types.
http://www.boeing.com/commercial/aer...icle_03_2.html