Well I'm not sure that love demands so much time. According to my experience that's enough a week to fall in love)) And it doesn't matter how many years you know a person...it can happen so sudden you just find smth that makes you love one
"Always" doesn't refer to how quickly Severus came to love Hermione, but to how long he loved her. If he has "always" loved her that means he loved her from the time they met (as he did with Lily), and when they met he was thirty-one and she was eleven. That's creepy.
I won't argue with your experience of love, because it is your experience. My experience is that "instant love" is a superficial attraction that doesn't last, while lasting love takes longer to develop. If lasting love develops from instant love, that's pure luck.
These words are taken from JKR book. It was told about Lily, not Hermione. He doesn't want to loose Hermione as he did with Lily that's why he is careful about the girl. I know it's poor explanation, but I see it like this!
Yes, I know the words are from the book. They are appropriate for Lily, but not for Hermione. That doesn't mean that Snape in an SSHG story doesn't love Hermione, only that he didn't love her "always." He would have begun loving her quite a few years after first meeting her when she was eleven.