lets say growth rate for a normal Discus
3.5 inches is general about 3-4 months old ( from fry not from when you buy it)
so at 6-9 months Discus should be at least 5 inches with proper upkeep.
at 12-15 months they should reach 5.5 inches and by 2 years they should reach 6-7 inches.
this is dependant on many factors all of which need to be followed to allow them to reach there full potential.