Longest I ever wrote was 78 pages. Right now writing one that will push well over 300 pages.
I never solidly wrote one, but the longest single fan-fic I read was Harry Potter and the Methods of Rationality.
The longest I have read had 38 chapters in it.
786 pages, 43 chapters.
Took me almost two years:)
I'm such a nerd!
Doctor Who "Random Chance" 61 chapters, 103,914 words