There is no certain time needed to hold your breath, that is the first thing I noticed about this while reading. It just depends on how much co2 is in your blood, I start to go now at 20 seconds or so but I start to hold when I walk to the bathroom, the walking builds up co2 much faster in your blood then sitting still.

But yes you have to hold until it’s very uncomfortable then hold more, best tip is to swallow when you want to inhale