A girl may say a guy is crazy if he says he likes her within 10 secs of seeing her. For me, I can like a girl within 10 seconds of first sight cos my focal point of attraction in a girl is just her facial looks. If a girl's face impresses me and her behaviour is bad to an extent, I can still manage her. But if a girl's face doesn't appeal to me and she has the nicest character ever, I wouldn't seek her attention.
What I have come to understand is, visual perception is the highest factor that decides who falls in love with who and theres no specific time frame to be used to decide how long it should take to fall in love. Love can happen in 10 years, 1day, 30 secs or maybe even as fast as 0.5 secs.