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This is a very difficult question to answer simply. It depends on what field in IT you are in, what your interests are, how you prefer to learn, among other factors.
IT is a very vast and fast growing field. No one person can know all of IT because there is just too much information to learn and it is constantly being updated and changed.
To better answer your question, you would need to specify which field(s) of IT you are refering to, e.g. networking, servers, clients, hardware, software, communication, etc.
