The correct answer is, of course, never. Unless you actually like wanna like come over like a gushing like teenage girl, yeah?
It's a frankly bizarre epidemic of faux affection. Hell, they've even appropriated - or misappropriated? - another letter to their overly-personal cause. Now 'o' means a 'hug'. So 'xox' means 'kiss hug kiss' and not, as we at first thought, a bizarre way of spelling 'socks'.
Back in the day, the 'kiss' mark would go into the other half's birthday and Valentine cards. Sure, you might add a 'kiss' to the bottom of a card to a relative or a good friend, but that was as far as it went. A single 'x' could suggest affection or love, intimacy or deep friendship. The current overuse though suggests one of three options: 1) You're a genuinely touchie-feelie person, 2) You're a complete idiot 3) Or you're the Whore of Babylon.
Because everyone's at it! It's bad enough when someone you don't know sends you one of those infernal smileys...you know, the :-) thing. The fact that your PC is trying to convert that into an actual smiley face shows what an epidemic that's become. The mind boggles, then, what the visual would be to accompany the current spate of kisses and hugs. A picture of Jordan, perhaps?
The time has come, good sensible people, to strike back. And the only way to do it is to go into the same world, but go in deeper. They send you a message with an 'x'? Send them one back and translate it as 'kiss' or 'peck on the cheek' or, even better, 'our lips brush gently and it hurts, this oh so brief taste of forbidden love.' If that doesn't make the beggars stop, or they're one of these heinous types who go for multiple x's and several o's, respond in kind with 'big snogs', 'wa-hey sexy!' or 'mmm, yes, and a lovely wet kiss back to you too.'
The irony is, of course, they'll stop texting and emailing because you're coming over like a weirdo. And that, we believe, is what they call 'win-win'.