My OD happened maybe 10 minutes after I did my 3rd shot. Did a half bag to feel well, didnt feel it, did a bag, didnt feel it, did a bag and a half (3 bags total). Woke up 5 hours later by a shot of Narcan. My mother found me, she tried screaming at me, shaking me, and even, and I quote, "slapped me across the face as hard as she could" (glad I didnt feel that lol), but ime, as long as someone can breathe and doesnt puke while unconscious, they are *generally* ok, even if they are out like a light. Position them lying on their side so they can breathe easy and not aspirate any vomitus. and ffs, if you have to call EMS/911/999, do NOT tell them it's an "[insert illegal drug here] overdose", otherwise police will show up along with the paramedics. Just tell them so and so isnt breathing, isnt responding to stimuli, and needs help immediately. Paramedics will come, you can tell them the specifics (they have Narcan on the trucks here), and if any illegal things are present, remove them. Yeah, saving a friend's life is a great thing, however getting him a felony conviction for an OD.. he'd probably wished he'd died, or you left him alone. I know I did...
Generally, if you OD it happens within minutes if IV'd, tho mixing drugs and ROA (e.g., drinking alcohol, snorting Xanax, shooting opiates) could have a "danger zone" of many many hours, as it's a dangerous combination. Do not leave someone alone, listen for labored breathing, check for cyanosis (bluing of the lips, face, other body parts), etc. IME, most fatal ODs on downers are caused by respiratory depression.