But there isn't necessarily a magic distance or length of time you need to be running to get a runner's high. The answer is similar to the question of, "When does running get easier?" in that it's different for every runner. Some runners may feel a runner’s high after their first 30-minute continuous run, while others may not have ever felt it during years of running. And, once you experience it, that doesn't mean you'll feel that way after every run -- you could go a long time before it happens again.
So, is there a way to "force" a runner's high? If you always run the same distance and pace, you may to switch things up and bump up your effort a little to start releasing those feel-good endorphins. Try mixing it up by doing a fartlek run or increasing your distance to see if that makes a difference. Sign up for a local road race if you've never done one before. Sometimes it takes the thrill of crossing the finish line for runners to really experience that sense of euphoria.
And, remember that even if you don't feel a runner's high, you're still getting plenty of benefits -– from stress relief to better self-esteem to improved cardiovascular health.