This topic has been discussed and I think there was a long thread on the Women's Issues section.
First, the cervix is not typically the deepest portion of the vagina. In most women, there is a pouch or "vaginal vault" that extends somewhat beyond the cervix, usually extending behind and beyond the cervix into what is called the "posterior fornix" (sometimes called the vaginal cul-de-sac). When women become aroused, the cervix often retracts towards the front and the vaginal vault "tents" upwards and back, sometimes for a considerable distance.
Secondly, in any given woman the "vaginal depth" depends not only on their state of arousal but also on body position. Some sex positions tend to shorten the vaginal canal, such as the missionary position in which the woman's pelvis is canted upwards and her legs are on the guy's shoulders. Furthermore, measuring vaginal depth depends on what reference point you use. In some women the "vestibule" (the part of the vulva between the inner lips but before the actual entrance to the vaginal canal) can be quite deep. In some women the labia majora (outer lips) are very puffy and prominent, so if you use them as the starting point the vagina will seem deeper.
In unaroused women the cervix is often not very deep and can easily be felt with a lubricated finger sometimes only a couple of inches in.
When talking about vaginal depth you also have to decide where the stopping point is. Does the vagina end when the penis or toy just touch the depth of the vaginal vault? Or does it end when the vaginal vault is stretched as much as the woman can possibly tolerate? Some women tolerate or enjoy being stretched and that is how they can accommodate the full length of huge dildos, enormous penises and fists. I can tell you from experience that not all women enjoy the stretching sensation, but it can depend a lot on their frame of mind, state of arousal, time of the month, and how the pressure is applied.
I have had sex with around 16 women (I'm now married). During that time my "insertable length" was just around 7 inches. I could feel myself touch bottom in about half my partners but typically only in positions that allowed really deep penetration. In a couple, however, I had to be careful not to get carried away because I could bottom out too easily and hurt them.
Based on my experience and what I have gleaned from the internet and talking to other guys, some considerably better endowed than me, I believe that the length of penis that most women can accommodate when aroused without getting stretched is usually in the 7 - 8" range although some are shallower and a few deeper.
