Vaginally I've never been tied beyond a few minutes. I think only anal or a seriously large dog and a smaller woman would be able to achieve any kind of proper 'tie' for any real amount of time.
People seem to overestimate how often a more solid "tie" happens with vaginal, especially men. Also, the dog's knot size relative to the snugness of the vaginal opening really does factor in too. Of course, other details can make a difference, and explain why it can sometimes happen only occasionally with the same dog. If no air or little air gets into the vagina, the knot went deeper that time, AND he is pulling back less that session, a vaginal tie will last more longer than usually, when he can otherwise plop out after humping.
Barney and Frankie were a pair that had an 8 minute tie on video. Barney was a bull mastiff with quite a wide knot. He also wasn't too active after stopping humping. He'd move slow. So, he'd stay knotted in Frankie much more often than what we see in other homemade vids.
The width of the pelvic bones can make a difference too, or the shape of it. Some women have a sightly more narrow pelvis up front. Depending on the position, they might more easily tie with a dog.
Also, it's not uncommon that the dog that has a huge grapefruit size knot that would stay lodged in a vagina longer ALSO has a shaft that is too long for some. This definitely happens.
Every now and then a woman might have enough vaginal depth for a mastiff that has a symmetrical penis. There were vids posted like that here in the last 2 or 3 years. She accommodated the whole of him, and he was definitely stuck in, after stopping humping, but within 30 seconds to a minute, with his slow and barely visible pulling, she birthed his large size back out. Could she have kept clenching and held the tie? I'm guessing that would have been too much clenching. Also, considering his huge length, him coming out might have given her some relief too.
There's just so many variables between the two lovers in how their sizes fit together, but also from one day to the next how much he'll pull, one position to the next, did lots of air get inside, etc.....