I think the correct word is "excitement." It fades as time goes by. But if someone choose to stay beyond the thrill, attraction, or lust, it develops into something deeper, something healthy. When you get to this stage where you don't expect and you accept the fact that this person will not always make you happy (no one can actually do that), but you guys make the team effort to make it work, then it can last a lifetime. There's no timer on that.