Which do you think is the mode effective time to implement a style guide: at the start of a project or after thousands of lines of code have been committed? I

Which do you think is the mode effective time to implement a style guide: at the start of a project or after thousands of lines of code have been committed? I don't see this as being dramatically different - establishing the ground rules for contributors before they commit a single line, so they can make an informed decision as to whether they want to be involved or not.