The
patch is 10.3MB. The game is not seriously bugged, but some people have run into serious bugs. I played the game half way through without it and didn't have any significant problems. Some of the fixes and changes are only applicable to new games, though. There are additional quicksaves added by the patch (to give you 2 by default), the inventory of a certain NPC was improved and various other fixes/improvements.
As with any game, I'd recommend installing the patch before starting to play.
You will not have a problem with Divinity being over too quick. It should take about 60 to 100+ hours, depending on whether you do all the side quests and how much you explore.