It really varies! Some people make a full-time living within just a few months, while others may take a year or more. It really depends on a number of factors, including the quality of your books, the marketing efforts you put into promoting them, and of course, luck.
That being said, there are definitely a number of things you can do to increase your chances of success. For example, make sure your books are well-written and edited, come up with an interesting and unique hook to help them stand out from the crowd, and make sure you extensively promote your books through social media, online ads, and other channels.
Of course, even if you do all of those things, there's no guarantee you'll become a best-selling author. But if you're persistent and keep writing quality books, eventually you should start seeing some success.