Depending on what types, some grow slower than others. You can "target feed" your zoas with a single mysid shrimp to each polyp once and a while also.
I got a 5-7 polyp frag of RPE from another member a year ago. It's now at about 12 polyps. On the other hand, some green zoas in my tank are growing so fast, I am fragging on a regular basis.
A guy over at uberfrags was just saying the other day that if you slice the tissue that new polyps will grow from the wound when it heals. It's a pretty interesting idea that I'm going to try with some of mine.
With the CUC I have there's no way I could target feed. I have to put my acan lords on a rack, out of their reach to feed them, so zoas are out of the question. Have you tried phytofeast or some other phytoplankton? I use that about every 3 days. I add it on the lower end of the reccomended dosage. They say 1-5 drops per gallon, I do 2 and I've had some great growth with my zoas.