Ok you only need to hold it in a few seconds for more of the cannabinoids to fall out of the vapor or smoke. There are always some left though. That happens pretty quickly and doesn't require any dwell time for increased parenchymal exposure.
Holding your breath won't hurt you. Too much CO2 and you'll simply pass out. That's why I'd laugh when my toddlers would tell me they'd just hold their breath and wouldn't I be sad ....... never worked. We have a heavy CO2 drive to breath. That changes if you have COPD you're having to use your Oxygen drive to breath but you'll pass out there too and breath assuming your medulla oblongata is still working

your breathing will kick back in.
Just a hesitation is all you need.