Here's a picture of a bleached carpet I bought a couple years back. Although it looked bleached I knew it was an otherwise healthy specimen. Sometimes an anemone isn't necessarily heathy enough to recover, so again it just goes back to the "it does depend on the anemone." But just to give you a timeline, this was the anemone when I bought it (May 12, 2004):
Here it is again approximately 6 weeks later (June 21, 2004):
Obviously the change was gradual, so it did not take fully 6 weeks to go from bleached to non-bleached. Afraid I don't really remember how long it took to notice a difference, since it was more of the nature of "it looks a tiny bit darker today than it did yesterday". I really should have taken a picture every day and then I could have done a time-lapse, that would have been kinda fun. Oh well.