Most poly's grow slow. There is no set growth rate for any species. I'm sure you've heard it a thousand times, but genes, water quality, tank size, diet, etc will all effect the growth of your fish. I'd expect slightly slower growth than a senegal, but you never know...
it really depends on the individual fish, one of mine grew like half an inch per month and me other one only has grown about a quarter of an inch per month, diet also plays a key role in growth rates