As fieldmouse’s link shows, it’s located at the end of the vagina. How deep that is varies among women just like length varies among men. Kojack10 has done much research on this and I need to do a search on his threads to see what he arrived at. When a woman becomes totally aroused her vagina expands. It’s said to take 20 minutes of her being aroused before the cervix tents (moves out of the way) and her vagina deepens. To me, this is a clear indication women do really desire more length when completely aroused. It makes sense that if her cervix is moving out of the way so that he can reach beyond it, then she is not feelingless deep inside like sex experts try to claim. There are plenty of guys, though not all, reporting that being able to get into the fornix has had very positive effects on women—-stronger orgasms, vaginal orgasms when she couldn’t before, intense facial expressions and/or vocals.
Though I need girth more, I’m going for length goal first. I’m not willing to take a chance on hindering my length gains. Thankfully, some girth gains tend to occur while working for length. I’m willing to look like a pine tree first, and then turn it into a mighty oak. Whatever it takes to be 10 bpel x 6.5 g. That can live in the fornix at will (perhaps not with a very small percentage of very deep women, especially taking into account all of 10 may not be in her), and scrub the g spot and clitoris steadily (except with some very loose women). A drawback from having that much length is grinding during intercourse will be impossible with some/many women. Another drawback is having to be careful not to go full steam ahead pumping her, at least not until she hollars for it :D .