According to a new study in the international Journal of Sexual Medicine, the best sex should last just seven to 13 minutes, and even three-minute sex is "adequate.

The sex study is the first to review what the experts believe is the ideal length of time to have penetrative sex, with the random sample of Americans and Canadians labeling seven to 13 minutes most "desirable".

Intercourse lasting between three and seven minutes was deemed "adequate", but anything less was "too short" and beyond 13 minutes was "too long".

The study, is designed to help calm couples' unrealistic beliefs that healthy sex should last a long time.

US studies show Americans expect penetrative sex to last between 15 and 20 minutes, even though self reports indicate it is over in less than half this time.

Lead researcher Dr. Eric Corty, from the Behrend College in Erie, Pennsylvania, said this was a situation "ripe for disappointment and dissatisfaction".

"In the fantasy model of male sexuality, men have large penises, rock-hard erections, and can sustain sexual activity all night long," Dr. Corty wrote.

"It appears that many men and women hold this fantasy. The results from the present study, by providing a realistic not a fantasy model of sexuality, are useful both in treating people with sexual concerns and dysfunctions, and, with wider circulation, in preventing the onset of sexual dysfunctions."

Dr. Jane Howard, a medical sex therapist from Brisbane, Australia, said "usually women are quite happy with short intercourse, and are not bothered about prolonging it at all, but nearly all men want it to be much, much longer".

She said it was important not to obsess over the length of intercourse, with time often suspended during the act anyway.