Here is a fun thing to try if you want to experience a long period of time in a lucid dream. As soon as you become lucid, search your memories for an explanation for your current situation. Keep trying to remember further and further back. When I do this, I find false memories stretching back infinitely far.

I think these false memories are what has made my longest lucid dreams possible. The line between memory and current experience starts to blur. It is like the entire dream happens at once and I am living my memories. Years can pass in a few minutes. This is most common when my dreams have a strong narrative. If I follow the story, the back story fills in and seems equally realistic. This is possible because, unlike reality, the entire dream is happening in your mind. It is all very close to imagination and memory to begin with.