Monocolus grow about an inch a month maybe a little more or less depending on what u feed them and ur wc schedule and tems usually grow a little more and faster cuz they grow a lot bigger in size
C. monoculus grow atleast 5 cm. a month in the first year, then they slow down.
In the first 12 months they should grow 40-50 cm. If !! you feed em well and give em a lot of fresh wather