Mine went from 3.5 and 5 to 6 and 7 inches respectively. I'm happy with the growthsweet! Mine's about the same too. I got him at 3inch back at the end of May. Now he's 5 inch. So I would say 2-2.5 for mine in the past 5 months.
The SB albinos have a certain appeal though. They look like marshmallows or the 3 blind mice.
The 4 marshmallows lol!