That's only about .33" a month, that's pretty slow.
Yeah, that does seem kinda slow! (I was going off of old memory there as I acquired my frontosa's in 2005).
I've just now gone back and referenced a different web site where years ago I had posted frontosa pics with size descriptions. All sizes are 'eye-ball estimates'.
I had purchased my fronts in Apr 2005 (I had described their size at purchase as 'barely an inch'), I'll go with a nominal 1.25" here.
Dec 2005: One kigoma is described as being 5" to 5.5" (I'll average that to 5.25"), or 4" in 8 months (0.46"-0.5" a month).
Feb 2006: This kigoma is described as 'approaching 6" ' (it was probably 5.75", estimated monthly growth rate 0.45").
I don't describe this kigoma's size again until Dec 2006 when I list him as being 7" or a little more (probably in the 7"-7.25" range; so since growth rate slows after 6" so at this point in time his average monthly rate from time of purchase drops to around 0.36").
This particular kigoma was the largest of the group during the years and usually had an inch over most of his peers, though he was eventually surpassed in size by one of the others (one front is noted at 12" by the time this kigoma reached 11").
Here's a link from another site where a frontosa owner takes monthly growth shots for nearly a year:
TR6's Images - Cyphos Photo Gallery