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cant really say. need to be more specific. do u mean lik in real time the length of ur dream. which is really hard to be accurate unless ur REM is timed in a lab and recorded. which by the way i was reading through guiness world records book and the longest recorded REM period was lik 3 hours and 20 min around that. guess that guy was having a good time. In dream time sum people have claimed to have lived in a dream for years. for me sumin lik that wouldnt be too long. like maybe a day or 2 in dream time.
i believe your question is not about the time you are in rem , at least i had some times really long and extensive “storys” and the sensation i was dreaming all night…
I can´t guive you now examples and much less can´t tell how long was the dream,but yes i had dreamt some “neverending storys”
I had one just recently… that I swear lasted the whole night. It was as though there were several dreams, because the story kept changing, however, it all happened at the same dream locale and they linked… even some of the dream characters stayed constant. Ive never had one dream keep linking to others all night like that before.
I took melotonin before bed though so maybe… shrug
Last week I had an super-long dream. I remembered every single detail, probably because I took 75mg B6 
The dream began at my home, then moved to my school and it ended somewhere in Siberia. Unfortunately I didn’t become lucid, but it was nice though 
I think maybe I have a tendency to overestimate how long my dreams are. I was looking at some old lucid dreams that seemed about 5-10 minutes long, and I went through them in my head, timing them with a stopwatch, and they were much shorter than I’d previously thought.
Maybe it’s just me, but when I hear people say ‘I had a lucid dream that lasted an hour and a half!’ I get slightly skeptical.
I had a dream last night that seemed to last many years (I was a teen in the dream when it started, I had grey hair when it ended), irl it probably wasn’t very long though.
I’ve had a couple that seemed to span a few years. I also kept half waking up and drifting back into them so they seemed especially long.