These likely aren't the longest per se
, but certainly among the largest
I've seen belong to one Azlea Antistia
. Images are notoriously difficult to find; most of what I've come across in searches are low-res rips of her videos.
Definitely worth the hunt, though - her nipples are certainly mind-blowing.
This pic gives you an idea of how large they are. I'm convinced they're even larger when she does a nipple job, which seems to be quite often in her videos. (Yes, they're big enough to do the nipple equivalent of a tit job on a guy.) Edit: Fixed the image post.