Longest I've written is a good 1500+ pages with one of my friends. It is still unfinished and requires massive editing. We might publish it one day, but I doubt it
And in case you're wondering how the hell we got so many pages: it took us about two years and a half for it to get that far, and sometimes we would write up to 20 pages a day each. That's 40 pages in a day. I've also written another dozen-odd unfinished stories with this same friend. Now that I think about it... this same dude is my best friend. I really got to know him whilst writing, guess that's why he's my best bud
I've also written a 140-page, unfinished story, which is the most I've written on my own.