There’s no good answer. How long it might take you to learn a new skill depends on what you want to learn, how deeply you want to master the skill, your experience and knowledge of related or more fundamental skills, and your personality. Learning to program might take 10,000 hours of practice. Learning a new language if you already know one or two should take less time. Learning how to manage a web server might take a few hours or a few months, depending on what you already know.
For any non-trivial skill, starting from scratch, expect to spend a lot of time practicing. Finding a mentor helps. Reading books and articles online helps but there’s no substitute for practice, preferably in something resembling a real-world environment. I find I learn new things a lot faster when I have something at stake, like a deadline, a contract, or my job.