Yes, afraid that it does take a while and concerned that one may be drifting in and out of consciousness throughout. Unsubstantiated concern, but still, it can take a while to completely expire.
It looks like vomiting happens at around 5 mins, whereupon you should take your second dose.
I've a plan to have the exit bag setup on hand for after that re-dosing point. If I'm still functional enough to operate it, I could render myself unconscious rapidly while the SN does its work. I may or may not need or be able to organise it after ingesting poison to kill myself, but it's a reassurance to have two methods on hand.
Plan is to take SN, wait 5 mins for potential vomiting, re-dose if necessary, apply exit bag if able/in distress, to facilitate unconsciousness.
Maybe I puke in the bag, maybe I pull it off accidentally. Neither is a huge concern in the larger scheme of things.
Either the inert gas ends me within a few mins, or the SN completes the job.