In 2007, "The New York Times" reported that it takes an average of 8.2 years for a student to complete a Ph.D., but you can shorten this process. Given that up to half of that time can be taken up with fulfilling course requirements, one way to speed things along is to apply any master's credits you may have earned to your doctorate. However, a myriad of policies determine whether you can apply these credits and if so, how many you can apply.
Universities often have different policies depending on whether you earned the credits there or elsewhere, so do not assume that the policies that apply to one institution apply to another. Even if you are staying at the same university, rules and regulations differ based on whether you are pursuing a Ph.D. from the same department from which you received your master's degree or a different department.
In many cases, the courses required for a master's degree and those required for a Ph.D. are the same. Consult the handbook for the program to which you plan to apply. If it does not answer your question, email the department for further clarification. Do not assume that your master's credits will apply to a Ph.D., even if they were earned in the same department and even if the courses you took would be the same courses you would take for the Ph.D. Many schools, however, do streamline the process of graduate study by building the Ph.D. program onto the master's program. For example, at the graduate center of the City University of New York, students enrolled in a Ph.D. program earn an en route master's degree for completing coursework and certain other requirements. These credits and requirements all count toward the Ph.D., but even if you do not continue beyond this point, you will have earned a master's.
When considering the transfer of master's credits from a different university, a bit more variance in policies exists. Some institutions either do not accept transfer credits, or only accept credits earned at the doctoral level. Ohio State University, for example, only accepts credits which would be part of a Ph.D. program at the previous institution, and thus, it will not accept master's credits. However, most institutions accept a limited number of transfer credits. The University of Missouri and Colorado State University both require 72 hours of coursework, and 30 of these hours may be master's or doctoral credits transferred from another institution. Wayne State University requires 90 hours of coursework, and up to 60 of these may be transferred, including master's credits.
Some schools do not accept transfer credits -- including master's credits -- wholesale but require a review process to determine if the credits meet their requirements. Sometimes, you must meet other requirements to transfer master's credits. For example, the University of Arizona will only accept credits for courses where you earned a grade of B or above. Check into the policies at the doctoral programs to which you are interested in applying. If possible, when visiting a program, whether at the same university or another, see if you can sit down with an adviser in the department to go over your master's transcript to determine which courses will be accepted and which will not. Generally, you must complete requirements beyond coursework, such as comprehensive exams and the doctoral dissertation, within the doctoral program. Even if you completed a set of comprehensive exams at your master's program and completed a master's thesis, these may not apply at the doctoral level.