It depends on a lot of factors. How big is your tank? How much rock is in it already? How much are you going to add? Has the rock been cured at all? Is it fresh live rock?
If you have a good amount of fully established live rock already and are just adding a piece or two the effect should be minimal unless the rock had I lot of die off.
If you don't have much rock and your adding twice as much and it is not cured at all and has a lot of die off it could trigger a complete cycle and cause havoc with your fish and corals. Due to corresponding spikes in toxic ammonia and nitrite.
Always safest to cycle it first or add very small amounts at a time.