Hugs follow a 3-second rule.

A fascinating bit from ScienceNow points to work by Emese Nagy:

How long does a hug lasts? The quick answer is about 3 seconds, according to Nagy's study of the post-competition embraces of Olympic athletes. The long answer is more profound. A hug lasts about as much time as many other human actions and neurological processes, which supports a hypothesis that we go through life perceiving the present in a series of 3-second windows...Crosscultural studies dating back to 1911 have shown that people tend to operate in 3-second bursts. Goodbye waves, musical phrases, and infants' bouts of babbling and gesturing all last about 3 seconds. Many basic physiological events, such as relaxed breathing and certain nervous system functions do, too. And several other species of mammals and birds follow the general rule in their body-movement patterns. A 1994 study of giraffes, okapis, roe deer, raccoons, pandas, and kangaroos living in zoos, for example, found that although the duration of the animals' every move, from chewing to defecating, varied considerably, the average was...3 seconds...The results reinforce an idea current among some psychologists that intervals of about 3 seconds are basic temporal units of life that define our perception of the present moment...the "feeling of nowness" tends to last 3 seconds.