Runners high is that euphoric feeling that you hear people talk about, when they go for a run or something that they feel after their run. It's something that you usually get towards a longer run. It's hormones that your body releases after going for a longer intensity. It's not something that you're going to feel, maybe after a ten or twenty minute run, usually it would be an hour, hour and a half. But again, it's not something you are going to feel after every run.
Yes, I have experienced a runner's high. It just feels really good. It's like a mind, body connection. You almost feel weightless. You want it to last for a while. You want it to last forever, but again, it's not something you feel every time, and it's not something that you want to strive to go out, because you might be a little disappointed. You probably won't feel it every time.
There's no set distance. There's no set time that you're guaranteed to feel the runner's high, but it's more so for a longer run. Maybe if you're training for a half marathon, you'll probably feel it at some point during your training, or maybe even during your race. If you're going to go out running for about an hour, hour and a half, maybe about six miles to nine miles, you'll start to get it over at some point. Once you feel it, it's one of those things once you feel it you'll know.