It will normally take at a minimum 4-6 months on a new tank for experienced reefers or over a year or more before it will support coralline growth, and sometimes it may never develop. The water parameters need to remain fairly stable and in the range for a mixed reef tank over a long period of time. Also be sure you are testing and dosing Magnesium (Mg) if needed as it like LPS and SPS utilize Mg along with calcium. Some LFS will scrape some off for you if asked, but even then it might not do much for you. I tried seeding several times with no luck. What ultimately worked for me was getting my Mg above 1150 and having it come in on snail shells and coral frag plugs or stalks of the corals I got from private sellers. It will go kind of like this, you're inspecting the tank and no sign of it. Then the next day you see a little purple spot on the back glass and you get super excited because you know the tank is doing really well. You leave it be for a bit then scrape it off and next thing you know it's all growing all over the place.