This is a subject that really intrests me, becouse if you could spend years in your dream world while it's actually a few minutes, then the effects on your waking life could be huge (for example, you could learn to juggle, or at least learn to do maths in your head way quicker.)

But there are 2 sides:

-Maybe it is possible, becouse some people have experienced/said to have had such long dreams, and dreams more often look longer then the REM period lasts.
Allso, time is relative, and if you are no longer tied to certain rules of your body and mind, you might alter how you experience time in your dreams.

-On the other hand, your nerve cells just can't work faster. If you would have to do 100 years of thinking in 1 minute. Your brain wouldn't be able to do so.

But slowing down time seems possible. However FEELING like you dream for days is something else, indeed like in movies and such.