That's a difficult question to answer. All the ones I've kept started out around 1.5 to 2 inches I'd guess. I would say that they took a couple of years to reach 4 inches or so. As
B
Belly up
stated, growth rate really isn't the first concern generally. I would advise getting at least 10 and keep in mind that they really need more room then you would think for such a small fish. A 75 gallon would be a good place to start. Make sure there are plenty of sight breaks, wether it be lots of wood or plants and such. The key is keeping them well fed, and giving the smaller or weaker ones a place to escape when things get rough. I've had success keeping corys and plecos with them, as well as pictus cats but haven't tried much else. Remember that they are lepidophagy feeders in the wild. With that in mind, they will target other species much larger then themselves. While this doesn't mean instant death for their targets, eventually they will wear them down, possibly keep them from feeding, and typically keep them cowering in the corner of the tank.
While I find them very interesting, it's because of the reasons above that I don't keep them anymore.
Going back to your original question, may I ask why you are so concerned with their growth rate? Please don't take this the wrong way, but it makes me think that you possibly aren't in a position to house them properly.