Cornsnakes and most other colubrids double in length, in each of their first two years. Of coarse this depends on how often they are fed and how well they eat. Out of the hundreds of snakes I've raised I've never weighted of measured any until they were near adult size. I've always thought that keeping track of how well they eat is the most important thing. So I'm guessing here but I'd say 12 to 15 inches for one that age. But as I said I would be more concerned about how well it is eating and how well it sheds.