Really depends on tank conditions and how much LR vs. base rock you start out with. Usually in a matter of 2-3 months you should start to see some visible growth and within 6 months to a year it could be fully transformed.
It will not happen overnight and you have to be patient as with everything else in this hobby.
This is what I did starting out as I did not want to pay the price for LR, especially when much of it is covered with other rock. If you are patient it can work out very nice and be very cost effective.
HTH,