There really isn't a definite time frame you just have to watch the levels. When you see Ammonia and Nitrites down to 0 ... at that point your Nitrates should bounce up and then you can do a water change to bring those down! At that point you should be good to add your LR
and just keep watching those levels.
You will most likely see an explosion of a fine brown algea also known as Diatoms, at this point you can go ahead and add some snails and hermits to help contain that.
What size is your tank? How much LR
do you plan on adding?