What is your water turn over rate and do you have a lot of coralline algae growth on the older live rock?
Providing the water turnover rate is high enough and the rock layout is open you should see a difference within a week to a month depending on the quality of the live rock.
If you have very heavy coralline algae growth I would suggest adding a sea urchin to your tank these feed on coralline algae and prevent it from swamping the live rock in some tanks coralline algae can be a real pest it scertainly would be in my tank if it wasn't for my sea urchin I'm forever scraping it off the glass.