Online Connection Required to Play All of Dragon Quest X
Only first few hours can be played offline, according to Square Enix support page.
During today's Dragon Quest X press conference, producer Yousuke Saito said that the game can be played 100% on your own, meaning you don't necessarily have to play with others. However, this doesn't appear to mean that the game can be played without going online.
Square Enix addressed the issue of the game's online system at its support page today. In a brief response to the question "Is it not possible to play without an internet connection," the support page says "It's possible to play offline for the first few hours after the start of the game. However, to play all elements of the game, an internet connection is required."
Precise details of the game's online and story components have yet to be shared, so we're going to have to wait for further clarification.
During the presentation, Saito and series creator Yuji Horii took the effort to assure fans that the game's online system will not be difficult to use, saying that you can easily log out, the commands are all familiar to past DQ games, and, as mentioned above, you can enjoy yourself fully without playing with others. They also mentioned that the game will have a huge story.