What title is the longest dojinshi than can be found FAKKU?

Who knows what the longest doujinshi is? But what I do know is that there are various tankobons (full volumes containing several chapters) on fakku. An example of one is Yanagida-kun to Mizuno-san.

Hopefully that answers you question.

Bibi Anbetsu has a lot of chapters (36 afaik), though very short and unrelated story-wise.

But I don't recommend you try to find it, I'm not responsible for any mental scarring.

That new Index doujin that they jus uploaded, 80 pages. They had to upload it in two parts.