My parents Picasso has gone from 2.5" to 3.5" in about a year and a half.
Nigers are much more liberal growers. I would expect a 'growth hump' like kjr mentioned, but I would think it would be higher then 5".
I have both a Niger and a Huma Huma in my tank the Huma was the larger of the 2 when I bought them last year at 2 inches the Niger was 1 inch a little guy. The Niger is now over 6 inches and the Huma is barely 3 inches yet the Hum still is the boss of the 2.