Half a day.
She texted me, asking if I wanted to hook up with her. Sure, why not? I thought, as she lived 40 kilometres away from me. Not taking it serious at all.
Later that afternoon, she texted me yet again, telling me it's over. We didn't even see eachother during the "relationship".
My shortest serious relationship (at least for my part) lasted for about a month. Apparently, I wasn't fun anymore.
EDIT: However, I'm still going strong in my current relationship which has lasted over three months so far. =)