Longest for me was six months, but there were a few gaps in the dream. Amazing dream, but for some reason I can't find the post anywhere...I know I wrote it down SOMEWHERE. Ack. Oh well, basically it was this really long survival-horror bit with a sideplot about me finding my brother's remains through a long line of clues found in jail cells he'd inhabited. Oh, and I got married. That part rocked. Well, the honeymoon anyways.

Basically, the mind speeds things up, and also removes lots of the more insignificant stuff. Think back on today, for example. Through the whole day, you don't remember every waking minute, only the more interesting or significant bits as well as the things you learned from those bits.
The dream shortcuts and only shows you those significant bits. Instead of acting out every minute, you'll just act out the general idea and your mind will tell you once you're all through that it's been ____ amount of time.