The duration of sex depends on how long you have been together and whether you are with a regular partner or cheating, according to the study.
Sex for married couples or those with long-term partners lasts just 12 minutes - five minutes of foreplay followed by seven minutes of intercourse, the same time it takes boil an egg with a hard yolk.
Sex for those cheating is twice as long on average at 24 minutes - 15 minutes of foreplay and nine minutes of intercourse.
Sex lasts longest for couples in new relationships - typically 30 minutes for each session in the first year of dating.
The results come from a new survey by IllicitEncounters.com, the UK’s leading dating site for married people.
It found that 62% of married couples or those with long-term partners complained about having ‘Groundhog Day sex’.
The time couples spend on love-making reduces by half in the first two years of marriage - going down from 28 minutes in the first year after wedlock to the average of 12 minutes.
Three-quarters of married couples (74%) admitted that their sex life was less exciting after they had been together for two years.
The vast majority of people having affairs (82%) start them because they are dissatisfied with their sex lives at home.
IllicitEncounters.com spokeswoman Jessica Leoni said: “Sex gets boring when you have been married or in a relationship for a long time.
“It is like Groundhog Day. Too many couples stick to the same routine every time and just want to get it over with in the quickest time possible - which averages out at 12 minutes for most couples.
"Sex is more of an obligation rather than something to be enjoyed.
“Love cheats tend to spend longer on sex because the relationship is fresh and they are keen to impress their new lover.
“The duration of sex in an affair diminishes over time as well - but at that stage the couple are far more likely to split up and seek a new partner.”