We might like to believe that true love can last forever, but those in the know reckon that it has a sell-by date of roughly seven years.
That's just seven short years between gazing at your other half as they sleep and arguing about whose turn it is to put the bins out.
Eighty-four months between snogging at the supermarket checkout and arguing over the dinner table.
Or 2,556 days between ripping the clothes off the man of your dreams and complaining about washing his pants.
Of course, reality will eventually intrude into even the most romantic liaisons. And no one could live in a loved-up bubble for years on end without losing their job, their friends and their grip on reality.
But now that so many couples live together before getting married - or don't bother getting married at all - the very notion of The Seven Year Itch seems oddly anachronistic, doesn't it?
After all, we associate it with Marilyn Monroe driving men wild in the 1955 film of the same name. Or more recently, whiskey-soaked Don Draper playing away from home while his long-suffering wife takes care of the kids.
We like to think we've moved on from all that and in away we have - now women are just as likely as men to scratch that itch outside their relationship.
According to UK dating site Illicitencouters.com, which helps people in relationships to have affairs, more than 40 per cent of its members felt the urge to leave their partner after seven years – but 'saved' their marriage by taking a lover.
Rosie Freeman Jones, spokesperson for the site, says: "The Seven Year Itch does exist and, increasingly, British spouses are turning to agencies like ours to cure it.
"After seven years, it appears the initial spark between most couples has waned, and in its place are the stresses and strains of keeping a relationship together, from financial worries to gripes about who's going to do the ironing."
So why do things go wrong at that specific point?
Probably because this is the time when couples have young children, are sleep deprived and stressed from juggling family and career. The result? They don't have the time, energy, or money for romance - after all, babysitters don't come cheap.
Another possible explanation is the theory that our lives run in seven-year cycles, so you could find yourself waking up to someone who seems very different to the person you fell in love with.
Of course, social networking sites and mobile phones have made it easier than ever to reconnect with old flames, flirt with strangers and manage an affair, or even two.
When you throw our notoriously short attention span into the mix, it's a miracle our relationships even make it to the seven-year point.
So perhaps it's time to adjust our assumptions about what makes a successful relationship and accept that it's normal to be tempted by the fruit of another.
In fact, this might even be the key to making monogamy work. Couples therapist Esther Perel reckons that the best way to safeguard your relationship is to introduce a little flirtation into your everyday life – whether it's with your partner, the office post boy or the cutie from the coffee shop.
So maybe you should retouch your lipstick before you breathlessly order that grande, double shot, extra hot, wet cappuccino...
By: Ceri Roberts