got my one at about 2 inches as well, and the other i picked up he was about 2 1/2 inches. now the smaller one of the two is like 4 inches and the one that was 2 1/2 is now 3 1/2 inches. nice growth spurt by my one puffer. probably depends heavily on water params, diet, tank size, SPG on how fast your GSP could grow. i have had mine for like 7 or 8 months