According to CAP168, in the UK the coded Twy centreline starts/ends at the perimeter of the ILS critical/sensitive area OR the lower edge of the side surface of the Obstacle Free Zone, whichever is farthest from the runway. So in practice this could mean when vacating a runway and the taxiway lights become all green, as seen from the cockpit, the tail could still be inside either of the above until the whole aircraft is beyond the colour coded section.