dreaming doesn't only occur in the REM stage,i even dream at the very moment i fall asleep and before i wake up from a night of sleep,scientists don't know much about the human mind and even less about dreaming,they are only beginning to understand all of it.
REM stage has the most brain-activity and the rapid eye movement,they are also often the most intense dreams
In one lucid dream experience i had attained lucidity right after a heavy REM phase where everything was black and empty,there was no dreaming,only my thoughts were there,where i suddenly realized i was asleep and woke up,that was my first lucid dream,because i woke up immediately i could remember a fragment of the heavy REM dream and wrote it down,lost the paper though,all i remember was lot's of flashy colors,brilliant patterns and strange sounds and echoes
I noticed there are many dream phases(REM phases?) each with it's climax(or maybe only one with a climax) and likely followed by calm blackness or a pause(only encountered one,or was it a strange empty dream?)i estimate there are five such phases,maybe more,maybe less
the dreams that occur immediately after falling asleep are different then all the other dreams and are calm and your mind is very passive,like watching a movie or something,at the very moment you fall asleep your mind starts to drift after a certain thinking-pattern that started while being awake(strangely replaced by thoughts that are seemingly automatic,perhaps likely the subconscious mind takes full control)
So anyway i think 1.5 hours seems realistically to be the longest possible lucid dream,time perception of course is greatly altered