The north Atlantic water vapor imagery shows that an exit to the east anytime soon is not very likely.
Note how the trough to Ernesto's right is filling in...I would have to think this is the reasoning as to why they expect Ernesto to left turn?
http://www.ssd.noaa.gov/goes/east/nwatl/loop-wv.html
With a High building in as well, Ernesto I would think would have a decent chance of strengthening substantially, once it regains the low level circulation. How Long, well it has a pretty substantial out flow intact so I would say that given a turn more to west, 8 hours before it is approaching hurricane status again sounds reasonable, Longer if it stays on this general NNW track.
-t