The results of a shareholders' vote on the merger of United Airlines and Continental Airlines are expected to be announced today. If approved, the merger would create the world's largest carrier. Meanwhile, observers say integrating the airlines' fleets, workforces, cultures and computer systems could take 18 months or more. They note, however, that customers won't notice much difference if the merger closes next month. "The bigger question," said Robert Shumsky, a management operations professor at Dartmouth's Tuck School of Business, "is five years down the road, will you have a very large, well-run company, or something that has devolved into the lowest common denominators of the two?"