It doesn't have to take any time at all. It can be as instant as walking up to her and opening your mouth. Hence the beauty of being direct. Until you do so, who knows how attracted she is to you.

Body language and voice tonality are probably the most important elements in a girl's attraction towards you. Anything else could be just fleeting, i.e. a girl may think, 'he's hot... until he opened his mouth' or 'until I noticed his nervous eye contact'.