For a CO2 tolerance test, you continuously exhale (a tiny trickle) to ensure no accidental inhale. See if that makes it harder for you.

When I was singing, I could hiss for 90 seconds without specific practice. For definitional purposes, a hiss was an unbroken, continuous, audible, 'sss' sound. This was an exercise our chorus director sometimes did with us.

I get bored more than anything else. Just did 70 seconds with one deep breath of prep. I have not sung in years.

Same. When I was a kid I would regularly go for over two minutes... I could get to three every now and then but barely. Haven't tried timing how long I can hold my breath in probably a decade. Unfortunately the site doesn't keep going so I tried with my phone, started squirming at 2:45 and made it to 3 minutes.

Nice! I was literally struggling just to hold it for 40 seconds. Of course, long ago I could easily hold my breath for 3 or 4 minutes. (A cousin of mine was able to do it for over six minutes or so. He could also dive to much greater depths than anyone else.)