Short answer is as long as it takes... but if its been 5 minutes I would move on the the soda water trick. Definately should come out after 10 minutes though.
As to it damaging the other inhabitants, it depends on what is on the rock. If yo have corals glued to the rock I don't think I would dip the rock. If you mean thecoraline algae, and bacteria that make it live rock I wouldn't worry, the coraline will come back and most of the bacteria is deep in the rock and shouldn't be affected by the freshwater