Depends on the size of the clit and the thickness of the vaginal wall. Best case scenario is large and thick wall - too thin and they get sensitive way too fast and can't get off. Larger clit mean more nerves underneath (the two legs that fork off of it underneath). Basically, women that get off vaginally = either an energy orgasm (all psychological - these women can get off on any type of sensual touching) or simply based off nerves that extend off the clit. The crazy thing is that every woman is different - the hardest to get off vaginally are the super small clits + thin wall (almost always they'll tell you they've never had an orgasm during sex and they're not lying). It's a major PITA and for those types I'd recommend just using a vibrator during sex if they want to get off guaranteed 100%, everyone's happy. This is coming from someone who's had a decent amount of experience.