This is an interesting question. There are two perspectives, one as an observer of a relationship, the other as someone actually in one.
When watching my friends, I see a serious relationship when they've been together for ~6 months, appear to show up to most group events and friend events as a couple, and leave as one too. When they're not together and you ask "Hey, where is *name tonight?" They know where the person is and why they couldn't come out.
When in one, for me it becomes serious not when the "I love you" has been said, but when we talk about taking the next real step together, ie meeting the parents, talking about moving in together, etc. If my friends were asked, they'd probably say I've been in 4, but in my books, I've been in 2.
"All is fair in love and war." - Francis Edward Smedley