It depends on what you mean by "build". If you want to set up the right files to make your website go online, then you could even do it in a single day by following a tutorial. Setting it up, instead, will take much longer whether you are a beginner or someone already well-versed in web development (you still have to get to know their HTML/CSS structure, after all). If you don't know anything about web development, stick to pre-built themes and customise details like colours, font family and size, you'll find plenty of free themes to choose from. Finding the right plugins and fixing the new issues that will arise as you go will probably take a week or so if you're dedicated.
If you want to make your own theme, though, it can take waaay longer. If you start from scratch it could even take an entire month, if not several, but expert developers who know what to do on Wordpress could take much less. It's still a daunting task, though, so don't expect it to be quick.