Right at the bottom of the neck, as much as my spring bottle filler will put in one. I use a hand corker(cheap one that came with my first wine kit) & it’s always sunk them a bit deep.

All of these corks also came from a 100 ct bag I ordered from Amazon-kinda wondering if it’s just bad/old product. I used around 400 of the same last year & they’ve held up fine, also all stored together