Whether you go for your Ph.D., Psy.D., or Ed.D....you are looking at a minimum of 4-5 years (plus internship and post-doc). You may get some classes to transfer in from your MS, though it depends on the type of program, the syllabi of the classes, and the institutional requirements of the doctoral program. Most of the time you are limited by when classes are offered, particularly in your first year. The classes are sequenced so you take your required classes first, so getting one class waived each semester should lessen your load, but not get you ahead in your class cycle. You may be able to take elective classes earlier, but most of the time people take that extra time and try and get a jump on their research.
A rule of thumb I have heard is that it could take off between 2-4 classes or about 1 class per semester for the first year or two if the course syllabi line up. Programs will most likely require you to submit syllabi before they can officially waive a class. There can be large variances in MA/MS level classes compared to doctoral level classes, so programs may still nix the waiver request.
I know from lecturing in some MA/MS classes that the material was not nearly as in-depth as what it was in the doctoral classes, though some programs offer classes that have both masters and doctoral students in them. I was told by one university that if they accepted a waiver for a class from their MS program into their doctoral program, that they would accept it from another university as long as the syllabi and required work was judged to be equal. The most common classes I have seen waived are Stats I, Adult Psychopathology, Child Psychopathology, and Research Methods.
There can be large differences in applied/clinical practice degrees and research degrees, as well as subjects within a department (mental health counseling, marriage and family therapy, experimental, clinical, etc), so be aware that a program can reject a waiver request for a range of reasons.
It seems that the non-clinical degrees have a better shot of being waived because the material is more likely to be in line with the doctoral level material, and often times are taught by the same core faculty. Applied/clinical degrees seem to be much more of a crapshoot, though obviously it will vary by program. I generally advise that if you are looking to do an MS first and then apply to a doctoral program, only go to research-based MS programs with a solid history of getting people into doctoral programs. I don't mean to pick on applied programs meant for licensure, but the coursework tends to be much different and offers less to strengthen your application, compared to someone coming from a more research based program. This applies to both Psy.D. and Ph.D. programs, as research is still a very important part of both.