depends on a ton of variables.. what kind of zoa is in question? some grow faster than others.. plus each tank is different (parameters, doc's, lights, etc..)
it's really hard to say exactly. but some will multiply faster than others just based on what they are..
I have some zoas that started to multiply rapidly (my orange bam bams took off and are about to double in <1.5 months). I have others that haven't really grown much..