Depending on the already existing skill level, learning web designing can take anywhere from few weeks to months; also, the amount of time one spends in learning and the person’s aptitude and talent, which may vary from one to one, would be contributing factors as to how quickly one can learn and start web designing.
For instance, it helps to know photo shop when designing a website, hence if someone knows photo shop already and is good at it, can save quite a bit of time and instead focus on what he does not know.
For beginners, learning HTML, CSS and Java Script is the usual way forward, and normally that should not take more than a month to get hold of the basics.
If the goal is to go higher, and get a job as a web designer, then attending a college for a course on web design is a option available, which can take anywhere from two to four years to complete depending on the course programme. For those unable or unwilling to join a college, can opt for online courses, and can complete the course in about a year’s time or less.