I am not sure the avarage time that a person spends in REM or in a dream state. But I think it is safe to say that the dreams that seem to last for a day a week are not true to real time.

I had a dream last night that could have been an entire movie. Or so it seemed.

So this is what I was thinking.
Could a mere thought of something |in a dream| be manifested has an entire dream scene instantaneously?
So in your dream the scene was never actually played out, but as your mind had rendereded the thought --- your memory had percieved it to have been an entire scene.
Making it possible to have a dream that is a week long.