First off, I love your avatar. Joker is my hero.

Second, it depends on how long the REM cycle you are currently in lasts. The ones in the beginning of the night can be as short as 5 minutes and the latter ones near the morning can be up to 45 minutes.

So, if you are used to lucid dreaming and can maintain lucidity for a full 45 minutes without getting to excited and waking yourself up, you can have really nice long adventure.