Question: what is the longest that a person can go holding their breath, safely.
It depends upon their volume and conditioning. Pipin Ferraras who held the world free diving record can dive down to 435 feet below the surface of the ocean on one breath. This requires tremendous practice and exercises to offset the breath-holding restrictions that it invites. My sense is that he has trained himself to lower his metabolism, heart rate and breathing rate at will. Something similar to what Yogis do when they are buried alive for several hours or days. This takes years and years to perfect. I counsel against breath holding for most people as it blocks the breathing system including the speaking and singing voice Sports Performance Program
To go much deep into breath holding I highly recommend Breathology by Stig Severinsen. It is a wealth of information that guides one in to state of the art free diving techniques.