As mentioned, a lucid dream can last as long as you are dreaming. Most dreaming does occur during REM but you can also dream at other times. I've read stories of people have (lucid) dreams that last 45 minutes or longer. However, as also mentioned, a dream can seem to last much, much longer. I have read stories of dreams seeming to last years. One way this can happen is if you keep convincing yourself that so much timed has passed that this becomes the reality.