The question of when you know you're ready for a relationship again is, like stacy said, something you know when you know (like being the One). I don't know about waking up one day and realising it, but certainly over a period of time your priorities change and you feel ready to jump in again.
I'd been happily single for over a year until around March I found the novelty of single life was wearing a bit thin, I don't regret a single moment but ... yeah, we're all human here (except the wookie

) and it ain't no secret that souls need to be nourished.
Last night I ended a nine month fuck-friend relationship (after testing the suspension on my car one more time), it was mutual and we both felt ready to move on, it was a sad and nostalgic seperation, not unlike a breakup but instead of reminiscing about walks in the park or a day at the picnic, we laughed about that time the swing broke or how she wanted to kill me that time when I was trashed and made passes at all her friends.
Just to add a final nail in the coffin, since that was my last fuckfriend, I've decided to abstain from sex completely until I'm in a comitted relationship. 8( One, because I'm over it and two, because I really don't like the role it's been playing in my life recently.
So I guess in a literal sense, I've known for a few months but it's taken that long to unravel and uncomplicate the last vestiges of a life that I spent quite a lot of time building. As for giving up illicit drugs, alcohol, nicotine, caffeine, renouncing Satan and joining the priesthood... maybe next week.
NB. This is not a comedown speaking
"She had it all in the bag so she should have been glad
But she was mad and sad and feelin' bad
Thinkin' about the things that she never had
No love, just sex, followed next with a check and a note
That last night was dope"
- Salt N' Pepa, "Let's Talk About Sex"