mine has grown a steady inch a month for 2 months now. They usually grow like this till about 4- 5 inches and then slow down A LOT. My last pair grew to 6 inches in 10 months. I have always kept them in "under" stocked tanks over 100 gallons with weekly 40 - 50 % water changes. I think it all just depends on filtration, water changes and quality food
My brother and I had a total of 3 sevs that got big..
They seemed to max out (or slow down a bunch) at around 8 inches if I had to guess. Maybe a little less. Mine took less than a year to go from 3 inches to around 8 inches.
The 3 little ones I have no are growning as slow as dirt. But... this has everything to do with feeding and water changes. When my brother and I had ours we changed the water every week, fed 2-3 times every day, ect. Now my tank is at my dads, and he feed on average 1x a day, rarely 2. And occasionally 0. And water changes are RARELY done... I may do them every two weeks if I go home... but they have gone months without a water change though before.
Also i have never seen one in person over 8-9 inches. I have seen people on the internet claim they get a foot long... never seen it though.