Well for most
men we realize it slightly after the first time we sleep with a you. Sorry, couldn't resist.
lol
I am not noted for my
longevity in relationships, so usually I'll realize pretty early on the that its not going to last but as long as I am having fun and still like being with the person I'll hang out.
Oddly enough, usually I'll realize I no longer love the person after they something silly or laugh a certain way, and then presto I see them in a whole new light.
But yeah I am feeling you about the whole light switch thing, suddenly on and then suddenly off.