How long does it take for you to really know someone?

I recently ran into a situation where someone made a quick judgment about me. Approximately five days later they told me how they felt. What is strange in this situation is that when I first met the person I essentially told them "I like the colour blue", then after 5 days later they said, "Well it seems that I don't like you because you like blue". It was just one of those moments that make you go huh.