5mg Heroin insufflated would likely make any non-habituated person very intoxicated for several hours. There is some variation from person to person, but that would be the majority of people.
Economics are going to be equally relevant when determining the total Heroin intake for a given region. You asked the question, "how much Heroin can a person use before it kills them?" In my experience, there is not a practical limit at which a person's tolerance to Opioids will stop increasing. I believe there could be some theoretical limit, but I have not ever heard of it or encountered it. In my experience, a person's tolerance to Opioids can grow infinitely.
Also, Heroin does not really exist anymore in North America, at least not as anything other than a "specialty" item catering to enthusiasts of the drug. Fentanyl, Fentanyl analogs and to a lesser extent, other potent synthetic Opioids (Nitazenes for instance) are currently what is available on the streets. I have not heard of Heroin being available in any significant quantity in North America for at least 5 years.
If you were on the East Coast prior to Fentanyl, it would be common for a person to use 1g-2g per day intravenously as a dedicated addict. If you met a person who was doing more, they were likely on a spiral or headed for ruin. If you were doing less than 0.5g per day, you were not yet a full junkie but you would be soon. I was always told ~30%-40% purity was not an unreasonable figure in and around Boston. The truth is, I don't know and I don't know anybody who does know. You can't rely on official numbers for anything.
I have also used Morphine/Heroin in the Middle East and in Southeast Asia. The Heroin in Cambodia was stated to be pure and I believe it was quite pure. It was clearly stronger than what I had used in Boston. Still, I ended up routinely injecting 3g per day, as I could afford it. If we are running the numbers, that could be equivalent to injecting 6g-7g per day of Boston Heroin. That should give you perspective regarding tolerance. It just keeps going up.
You're doing research. Something you're never gonna hear people talk about, is that unlimited Heroin is not even enjoyable. You think it's gonna be the greatest thing ever; everything you've always wanted. When I was injecting that much Heroin, it made me feel numb emotionally and physically to such an uncomfortable extent that I found myself routinely screaming as loud as I could to convince myself that I was still alive and existed. I would use more Heroin in an attempt to get rid of that feeling. My body and mind had adapted to understand Heroin = Good; More Heroin = More Good. When that equation broke down in my mind it was a pretty unsettling experience.
Still, once I reduced my usage and was using 0.5g-1g maximum of the SE Asia dope per day, a lot of the positive feelings returned and I became more functional. By the time I left, I had learned that using any more than 0.5g in a day led to a significant lessening of the positive effects of the drug. This is something you can only learn when you have free access to the drug with essentially no consequences. It's only then that you can freely investigate and interpret your relationship with the drug, not the relationship with you, the drug and your money or you, the drug and the legal system or you, the drug and your girlfriend; just you and the drug.
So to put a bow on this, a person who is using Heroin the "right" way is going to be using the lowest amount possible. This is not because it's safer, cheaper or because it's the right thing to do; it's because this is how they achieve the greatest cost/benefit relationship with the drug. For the record, even that 0.5g of Cambodian Heroin I'm talking about would likely be way too much for most people and was probably still a little too much for me.
