The growth rate of zoanthids varies so greatly from tank to tank and zoa to zoa. Some grow so fast you may have 50+ polyps in a couple months or less, others may take a year to get 50 or even fewer. If you want them to grow quickly, just keep a stable environment and don't move them around too much. I think on average a most frags that size would at least double in a month. Fresh frags tend to muliply rapidly at first as long as they are healthy.
