I've had/have 3 Severums. In my experience they grow slow to 3 inches. Then they have a spurt from 3 to 6 inches. After that they slow down again as the reach their final size of 8 or 9 inches.
It's hard to pinpoint an absolute timeline, but I grew my Gold Severum from 1" to 4" in about one year, then after another six months it reached 6". It topped out at about 7.5" after two and a half years.
Just keep up with water changes, and feed them a high protein diet. Use quality pellets as a staple. Severums grow to be thick, tall bodied fish, so they look a lot bigger than the length would imply. They are also tougher than they look, so you should have no problems keeping a smaller Severum with a bigger Oscar, as long as it can't fit in it's mouth.