Basically three years. You only need to pass the doctoral entrance examination to study PhD. First of all, we need to choose schools, directions and tutors, and we need to foster strengths and avoid weaknesses. Three years should basically be chosen according to your own interests and specialties. You must contact the tutor, at least let him know that you are taking the exam; Then study the ideas and difficulties of schools and majors (including foreign languages). Finally, collect the examination materials and review them according to the data and analysis. Some schools are chosen by tutors among students who have passed the preliminary examination, and some are decided collectively by tutors. But in any case, it has been recognized by the tutor, an academic recognition. In addition, it should be said that illegal law study and master's degree are practical, and doctoral degree is research-oriented. There are differences between the two, but there are also similar and overlapping parts, such as the research object.