So I've been seeing this around a lot lately in various articles and "amazing fact" lists and even found one place that gives some sources PositivePsychologyNews but I can't seem to find the original source of the 20 second claim. The closest I found was in this study referenced in the above article which states... (emphasis mine)
Couples were seated on a loveseat in a quiet room and instructed to sit close together, holding hands if they felt comfortable doing so. They were asked to talk about a time they had spent together that had made them feel closer as a couple (2 minutes). They then watched a 5-minute segment of a romantic video they had previously seen. They then were instructed to talk for 2 minutes about a time when they felt close as a couple. During this time, couples were unmonitored and unobserved except when the experimenter entered the room to give instructions. At the end of this session, partners stood for a 20-second hug.
Perhaps I'm misunderstanding, but that seems like the 20 second hug wasn't isolated as the cause alone. Have there been any other studies that suggest the 20 second time frame? Did I miss something in that one? Some of the sources listed are behind a paywall sadly.