It takes an average of about 20 hours to assemble a road bike from scratch. This includes the time it takes to select each individual component, as well as the actual assembly process. Of course, this will vary depending on your level of experience and mechanical ability. If you're new to building bikes, it's probably best to allow for more time just in case.
Assuming you already have all the tools and components you need, the actual assembly process should go relatively smoothly. Start by attaching the fork to the frame, then install the handlebars and stem. Next, add the wheels and tires followed by the brakes and drivetrain components (crankset, chain, cassette, derailleurs). Finally, finish up with the seatpost and saddle.
If everything goes according to plan, you should end up with a brand new road bike that's ready to hit the pavement!