I realize this post is a few years old, but this is one of the first things that comes up on Google when you search for this issue and perhaps my experience will help some passersby.
I recently ran into a situation where I was animating a plesiosaur in Cycles at about 16 samples with motion blur turned on. It was taking between 2 and 10 minutes per frame (depending on how dramatic the frame-to-frame movement was).
On the very last frame, the estimate suddenly jumped up to 7 hours, with no discernible reason as to why. However, upon further inspection, it suddenly became clear that I had made a frame AFTER the last frame... wherein the plesiosaur was in an extremely different position. In other words, the motion was no longer continuous, so Blender was calculating overtime to try and figure out how to render such an insane motion blur.
Bottom line, if anyone does run into this issue, try to make sure none of your objects suddenly have an extremely dramatic frame-to-frame movement, because that can slow things down to a snail's crawl when motion blur is turned on. The more continuous your motion, the better. Hope this helps any passersby! Peace and God bless!