^Depends on how high you wanna get. Constant puking is an early sign of OD is you're sniffing it, but a lot of people who don't have a high opiate tolerance puke from non OD amounts as well. Losing consciousness and turning blue are surefire signs that you're ODing, but if you're alone you won't notice that it's happening to you and be able to do anything about it anyway. Especially if you IV heroin, there's really no way to tell that you've done too much and be able to seek help in time. snorting it, you have a small window that you may have time to do something about it. Most people snort a few...wait, snort a few more...wait. If you're not nodding too hard and throwing up, you may want to do more, you may not.
The only thing you can do is start off with less than you think you need to get high and re-dose very carefully and responsibly if you're worried about ODing. The question of "How much is enough" can't really be answered. It's like asking "How much whiskey should I drink?"....Well, obviously not so much that you're seeing double and throwing up, but other than that, some people like to have a few shots and call it quits, some people like to get completely bombed, it's completely subjective!
The big difference between alcohol and heroin is, there's a much smaller gap between using enough to get high and using enough to OD than with many other drugs. A little bit too much can cause you to lose consciousness, stop breathing and die.
You should definitely use heroin with a "buddy", particularly if you're unsure of the purity and how much you can handle!
It really does come down to using common sense, and even then, there's no guarantees. This is why prescription opioids are safer in some ways, you can have complete control over your dose and always be aware of the exact amount you're using.
