Dude, you'll know the right time to do it. I mean you should, like tell her she's hot then lean in and kiss her. Something like that. If she's more refined and you barely know her, tell her she's cute. Whatever. Generally from there on it's all electricity and you should be making out all night. Of course be confident and make sure to compliment her now and then, but don't be like overly persistent to where you're like constant just to where you are obsessive just pamper her a bit. Remember approach it with confidence. Alot of guys have trouble with that.