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There are a lot of factors to consider here.
1. My penis varies in the length, if I am turned on a lot, I did once measure almost 7", though that was few years ago.
2. Vaginas vary a lot, I once shagged a girl where I didn't even touch the sides let alone the bottom. But most women I do just touch the bottom, I think, however my current lady is very shallow and definitely bottom out.
3. The sexual position makes a difference, with her knees touching her chest the depth is reduced dramatically.
4. Then there are life's anomalies. years ago I was working as an engineer at a porn shoot in Belgium (the mind boggles) and there was a "big" guy lying on his back, ready for a scene. He was the biggest I have seen, by a long way, close to a foot long and as fat as your wrist. Without hesitation a petite 5 foot blond girl walked over and sat on him, taking it all in her pussy. I almost expected it to come out of her mouth.
So there you go, there is not one answer.