You dont have to wait 6 months at all. In fact, i got a frag
from a friend with coralline on it and its already spreading after only 2 weeks. What you need to do is get some coralline from a friend or local LFS
and then put it in front of a powerhead in your tank. To just try and start coralline by itself from a normal tank will take a while thats why having some introduced will speed the process greatly. As far as why you arent growing it now, what are you calcium levels? Do you use Strontium? If you cant get coralline locally try Seachems Reef Builder, ive heard good things about it. Also in your next post tell us what lighting you have.