Cool, your question is kind of sliced, the growth rate always depends on the variables. Tank size, feeding, water quality etc.
So what you going to do with the tern once you get your hands on a fully grown one? You got a shoal already or what?
To be honest , it's just my favorite fish. On the old predatory fish board there was a guy named John ( Knifeman ) he had a huge one called Bubba I think. He was so cool to see, it just really caught my eye. I am in the process of building a plywood tank that will eventually house a large mixed shoal.