Wayland, corals can recover from brown out anywhere from two weeks to two or more months depending on the type of corals and type of stress, if its is a frag then you can tell about the speed of the recovery at the base of the coral where it is encrusting, thats the first spot to see any color and the second place is at the tip where the new growth is coming out(this is generally slower). If you are getting new growth at the base and the tips then the corals are well on their way to recovery and color back up in about 2 weeks as long as they are not stressing out by other suddent changes in your tank. This is the experience that I got from my tank, your tank can be different so the recovery time could varied from tank to tank.