Sometimes a saltwater tank will "insta-cycle" when a lot of live rock is added. The live rock will act as the biological filtration for your aquarium. If you add a lot of healthy, established live rock, your tank should have enough beneficial bacteria to be instantly cycled. I do recommend that you wait a few days to make sure there aren't any unexpected spikes in your parameters (like ammonia) after adding the rock, however.
Some people use bacteria in a bottle, but I am not completely confident in that method yet, therefore I can't recommend it.