I've come to learn it depends on tank size, feeding and most importantly... water changes.
had a Xingu I in a 30 and 55, never seemed to grow. now that's it's been in the 125 for a yr, it's gone from 3-4" to almost 9".
Cincta was in a 150g since 3" in nov '05, nov '06 it was over 10" and now pushing 16".
Johanna went from 5" in a 55g to 10" in a 250g pond in under 6 months.