The quick answer is until you have a zero (0) reading for Ammonia, Nitrites, and Nitrates. Then it is safe for livestock.
However some corals will not do well in new tanks and should only be added to established systems (6-12 mos minimum)
If you can give us some ideas of what kinds of corals you intend to keep, we can help you out further. You may also want to pick up a copy of The Conscientious Marine Aquarist by Robert M. Fenner. It is a good read and easy read, but full of information for the new aquarist and there is a new edition out.