I heard a story about a monk that dreampt for 100 years (the dream lasted 100 years), not sure if it's true or not but it would be pretty cool if it was. I also saw a thread on this forum (or maybe it was another forum) about a guy who dreampt for 5 days strait. He would wake up in his dream and go about his business until it was time to go to bed again then he would wake up and live out another day in his dream.

My longest lucid dream was probably about 5 minutes long, if that and most of that time was spend looking for a place to have sex, the dream ended, as it usually does, when i had found a suitable location.