There's no real way of giving a time frame. It will depend on how quickly the bacteria colonies grow on the base rock which in turn depends on lots of factors such as temperature, feeding, lighting, type of base rock, etc.
What I do know is that you'd be better off saving the money you would spend on base rock and buy only Live Rock, it will mean your tank will look a bit emptier but it will create a far better tank later on. A reef takes a long time to build, create & settle.......you cannot do things quickly and this is not the hobby for impatient people. You will have to resign yourself to the fact that your reef will not look its best for a long time to come. My reef is just over 12 months old and is only now starting to settle and look the way I want with the right amount of rock, etc.
HTH