well my father use to own a shop and i heleped build up bikes from time to time, and out of the box they are easy to build.
mainly the derailers, brakes, headset/handlebars...ect all need to be adjusted or put on along with pedals. but small things like grips..ect vary from brand to brand. but mostly the whole bike is fairly built. should only take you around 30 min to an hour to do.