1" per month when were talking parachromis till about 9" or so is a very good growth rate for a male. When spawning may be a little slower and females usually will grow slower than males but not always. also depends on stock in the tank and dominance of the fish and most importantly water quality. parachromis need lots of clean water.
I totally agree with fishman09.
I attribute the growth rate in mine to daily 25%-40% water changes.
The alpha male grew much faster than the others, the female remained about 3" smaller