Considering corns are ready to breed at around 300grams in size then I suppose that would be deemed adult in females, but a lot of people advise to breed when they are 3 years old, so I would count mine adult at 3 years and weighing around 300g.
According to The Corn snake manual some corns can be considered adults at a size of 30 inches whilst others grow to 5 - 6 ft.