Normally for adult head/shoulder shots how far can you get from the subject when using FF?
A good distance for an adult head/shoulders portrait with the 135L on FF is about 8-10 ft (2.4-3 m). The reason 135mm is the 'classic' head/shoulders portrait lens is that distance is just right for a flattering perspective, for not being 'in the subject's face' and not too far away for communication without raised voices. Perspective is important for portraits, and it's determined by distance only, not focal length. That ~8-10' distance is key, consider the other 'classic' portrait focal lengths - 85mm for torso, 50mm for full body, 35mm for environmental (full body plus location), all of them frame the subject at that same 8-10' distance.