Not GP, but have trained freediving and used to spearfish almost daily. There is usually an easy period until the desire to breath starts, from 1 to 2 minutes

Not GP, but have trained freediving and used to spearfish almost daily. There is usually an easy period until the desire to breath starts, from 1 to 2 minutes in. The next minute or so is tricky, you get contractions as your body tries to breathe. After that you start to slowly zone out, but you're no longer in pain. Eventually the world closes in around you. When spearfishing you're typically fairly conservative and never get past the first easy section.