I could compare the PhD degree to a quest--there is a prize at the end of the difficult road but for most of the journey, it is you against yourself. There are tests you have to pass and challenges you have to overcome to get to the next level, but unlike the degrees you have earned upto now, the deadlines for a PhD are flexible and are mostly for you to determine and set at your own pace.
But how long does it take to get a PhD? A lot depends on how your research is going, especially in science subjects. Are you lucky enough to be able to gather adequate data on your research area in your fieldwork? Is there enough literature in your research area to help you set your direction? The early success or failure of your original research experiments would also determine the final time it would take for you to complete your PhD requirements.
Whatever your stream, department, subject and topic, the PhD program is intensive and this period of your life will probably ask you for your hardest work. If you’re like most graduate students, you will probably support yourself and your education by working as a teaching or research assistant in your department. Those additional responsibilities will also naturally add some time to the total taken for you to complete all your requirements as a PhD student.
Once you've been accepted into a PhD program, you need to complete about 2 years (between 60-65 credits) of coursework. This is the only part of the PhD program that you can plan and schedule like your undergraduate and master's degrees.
In the meantime, you must choose your advisory committee and begin studying for your comprehensive or qualifying examinations. Usually, studying for your comprehensives will take about 6 months along with your additional responsibilities as a teacher or researcher. The time taken to study for and schedule your comprehensive examinations depends on your own pace and confidence.
From here on in, you will have broad guidelines and time-limits given to you by your department, your major professor or your university but will have the leeway to schedule the rest of your program at your own speed given the ultimate deadlines.
Once you’ve taken your written examinations and passed them, you will probably have to go through an oral defense of your written examinations and a discussion about your research topic with your advisory committee.
Two and a half years are now gone.
The next step is to set up a dissertation committee, write a research proposal which includes the scope of your project, the literature you will review, the questions you will address, your dissertation blue-print and the time you think you will take. This proposal must be passed by your dissertation committee.
Add another semester and three years have passed before you’ve become a bona fide PhD candidate.
Now you will undertake the majority of your research and write your dissertation under the supervision of your major professor and your dissertation committee. Finally, once your committee is satisfied with the quality and quantity of your research and writing, you will defend your dissertation.
This last stage depends completely on you, the research you undertake and how that research satisfies the benchmarks set by your dissertation committee. It can take anything from between a year to many years.
The earliest I've personally known a PhD degree to be conferred is 3 and a half years after matriculation--and let me tell you that is very, very rare. At the other end of the spectrum, I've known a biochemistry candidate being awarded his PhD only after 10 years of being in the program. On an average, however, your PhD will take no less than 4 years and shouldn't take more than 6 or 7 years from the date of enrollment.