There is this event that occurs on movies as well that might be somewhat related to experiences where people say their dreams lasted for hours. I'll narrow it down for you:

- You're watching a movie scene where the hero and his beloved one kiss under the stars at night
- Then, you see them married and with a kid in a sunny day at their house

What you do here is pretend (even though you aren't aware that you're doing it) that you believe that a very long length of time has passed. Imo, this might happen as well in dreams. Besides, other factors probably come into play. Take people that are extremely focused on being lucid. They do a RC every minute, keep noticing things in order to maintain themselves lucid. That means with so many interruptions the dream seems to last for ages.