Same thing Curby said the K - to many variables to be precise .
All though the commons I breed usually go to the shop about 6-7 months
of age @ 3.5-4cm(which is also a variable measurement) , at about 12 months
they are a "solid" 5-6 cm. So with that in mind I'd guess at 24 months old you'd
be looking at about 10-12cm.