An understandable question…
This should be recognized immediately as a counter-productive question to be asking yourself. In the same way as asking how long it takes to learn to play piano does not help your progress in any way, and is more likely to lead to despair and lack of motivation if the results do not happen at the pace you hoped, so too with Astral Projection!
Learn to enjoy the learning process, and be proud of the small steps that you will make towards being able to induce an OBE at will.
This, incidentally, is a good motto to use when learning any new skill!
You will learn in different ways from other people, and you may find that some areas will come more naturally to you, and some won’t.
Relax, it will all come together in time.
Nonetheless, if you are asking yourself this question, there is a likelihood that this will not be a satisfying answer, so I will try to give a little guidance as to how long it may take to become competent at Astral Projection.
Virtually nobody “gets” Astral Projection first time. It is a process that can seem incredibly alien to most of us, so any claims that you will be able to learn the techniques behind successful OBE, without help, in less than a few months are generally unrealistic.
You may get a few instances where it works, but these will be caused more-or-less by a combination of chance and a kind of beginners’ enthusiasm, that will be hard to reproduce on a regular basis.
Actually mastering Astral Projection is really only possible when you fully understand each stage and how they tie together to create the right conditions. In fact I know one person who, in spite of some early successes, took a year and a half to start to get predictable results.
So keep trying, and I have full confidence that you will master it eventually!