Cleo is roughly 10 months old, and I've been free feeding her since I got her. She was literally half starved when I got her, so she ate and ate and ate some more for a while. While I'm still basically free feeding dry kitten food, I actually measured out what she was eating just to see how much food she goes through in a day--about 2/3 a cup of the dry, plus she'll eat two or three mouthfuls of canned Iams a couple of times a week.
The vet said I should start the adult food in September, as we're really guessing at her age. She's not big and probably never will be, and is still on the thin side, so even if she's really over a year old then, the extra nourishment probably won't translate into a tubbie cat.