Lots. Some people have almost no wiggle room, from day one. (I sometimes turn potential witches whose talents as humans are non-magical myself, so I know this.) Some people like Elspeth are extensibility prodigies. How fast they settle also vary - it seems to matter how often you use it (more is better), and how you feel about its associated personality trait (the broader the principle and the more central to the personality, the better), but obviously that isn't the only factor.