In response to:
How long does it take to know you are in love? One week, one month, one year? I think if it only takes a week it is lust not love. Any opinions?
What is love at first sight?
Does it exist?
Is it too early in the day for this?
To me, it depends on how you define love, and everyone is going to have their own definition.
But without overanalyzing this *too* much...
I believe in soul agreements and believe that we have soulmates and "harmonious" souls, so when we meet them we immediately either fall for them or hate them.
William and I knew the first time we laid eyes on each other that there was a significant connection, and he says he knew right away that he was going to marry me. (we actually started dating some months later, we knew the first week that we would eventually get married, I just had to work through some of my fears about that (and get my divorce finalized

) before I could really move into that head space.)
I've heard other couples say this too, that they knew right away. I wouldn't call that love though, not yet. I had already come to love William over the months of getting to know him spiritually, intellectually and emotionally as a friend, and he felt the same way about me, before we ever started dating. What was left was spending time together to learn about each other day to day, and fortunately there were no big surprises there!
So... no answer. I've fallen in love with people over months, or very quickly.