There are a lot of news stories about prescription opioid suicides. I don’t recall seeing any stories about how long a time between ingestion of the lethal dose and death. I assume some are time release and take a longer time. Do we know long a typical successful suicide attempt lasts?
I believe the majority of stories are not about intentional “suicides”.
There has been a tenfold increase in unintentional overdoses due to the huge number of narcotics prescribed in the past 10 or 12 years.
As to how long, depends on many, many variables. What drug, how much, who took them, empty stomach or not , just to name a few.
The route of administration matters too. If you inject it, you’ll nod off in a matter of seconds, but it takes a while to die from respiratory failure. Whenever you hear about some rock star being found dead with a needle in their arm, they actually died from an anaphylactic reaction to something it was cut with. The nasal route can produce rapid results too.
One of my classmates died from a fentanyl overdose that was ruled accidental. He was found unconscious at his home with 2 Duragesic patches stuck to him, and the person who found him, who was unidentified in the story I saw, said he was “an avid user” of that and Vicodin. 
The story said he was pronounced dead in the town’s ER.
Death from overdose via the oral route depends on whether immediate- or controlled-release tablets were used, or if they used liquid. I don’t know how long it would take to die from an OD if one used the rectal route; yes, these drugs can be used that way, and morphine, hydromorphone (Dilaudid), and opium suppositories have been commercially available in my professional lifetime. And of course, there are the goofs who like to take them medicine SOME OTHER WAY…:dubious: