Assuming you have a basic understanding of SEO and how it works, then learning keyword research shouldn't take too long. A couple of weeks should be enough time to get a grasp of the basics.
Of course, the more time you spend on keyword research, the more proficient you will become. And if you want to really master keyword research, it could take months or even years. But if you're just starting out, a couple of weeks should be enough to get the hang of it.