I'm so tired of hearing that the female vagina is only maximally 4-5" in depth. Any guy who has a cock with an insertable length of 6" or more who has screwed a few chicks knows that this is bullshit, yet it gets repeated ad nauseum.
A vagina can still be 4" in depth, but still able to insert a 10" dildo in to. Vagina's aren't a static size, they're more like those balloons used to make balloon animals. Measuring their 'maximum' size is a bit silly: the maximum size of a balloon would be as big as it can get right before it pops. Clearly such a metric makes no sense when talking about human organs.
Where guys get really confused is that they assume all women are built and respond to sex in the same way. There is actually an incredible amount of variety.
The anterior fornix (front of the cervical opening) is where a _lot_ of women (in my experience) "bottom out", because it's extremely sensitive and painful a little beyond there. However, for a sizable portion of women that area feels really, really good. Again, just going from my experience, but the best deep spot to hit is there (usually called the a-spot), and if my dick head is feeling that hard bump on it towards the end of my stroke, it feels pretty awesome for both of us.
Some girls can take it a little beyond there, but if my dick's angled just a little bit up they get hit head-on to the cervix, which very few women are in to. In my experience, that has always just caused the sex to end.
Then you have the posterior fornix (which is on the cul-de-sac, CDS), which is right at the literal end. I've been with very few women who really like that getting worked, and most can't really even feel past that point. I suspect it's because the clit/gspot/aspot/cervical opening are all being pushed against by that point, and since almost all women are pretty sensitive on at least one of those areas their mind is usually there and not the cds. That said, for those few women who do like it (again a tiny minority) it can be a really easy orgasm trigger. My problem with it is that I don't really get much out of it.. a rough, tight push against the vaginal opening, or a long stroke across the aspot + gspot feels way better to me. Their tightest part (vaginal opening) is right on my least sensitive area, so I'm mostly just feeling hard cervix pushing against my shaft, which is okay but not amazing.
So in short, there are a
lot of places that can be considered the 'end' of the vagina, and since the whole thing just stretches out it's not something that can be measured in any practical sense. The point at which discomfort occurs is generally considered the end, and that varies so wildly from woman to woman.